Reducing a drop in a residual supply-signal level on start up or restart of a power supply

ABSTRACT

In an embodiment, a method includes generating a pulse-width-modulated signal having a duty cycle, and isolating a power-supply output node in response to the duty cycle corresponding to a signal magnitude that is less than a magnitude of an output signal on the power-supply output node. For example, where a power supply has a non-zero residual output signal (e.g., output voltage) on its output node after the power supply is deactivated, a power-supply controller can use such a technique to reduce or eliminate a drop in the residual output signal caused by, or that would be caused by, a restarting of the power supply.

PRIORITY CLAIM

The present application claims priority from copending U.S. Provisional Patent Application No. 62/217,475, filed 11 Sep. 2015, and claims priority from copending U.S. Provisional Patent Application No. 62/272,419, filed 29 Dec. 2015, which applications are incorporated herein by reference in their entireties.

CROSS-RELATED APPLICATIONS

The present application is related to copending U.S. patent application Ser. No. ______, filed ______ 2016, entitled REDUCING A DROP IN A RESIDUAL SUPPLY-SIGNAL LEVEL ON START UP OR RESTART OF A POWER SUPPLY, which application is incorporated herein by reference in its entirety.

SUMMARY

In an embodiment, a method includes generating a pulse-width-modulated signal having a duty cycle, and isolating a power-supply output node in response to the duty cycle corresponding to a signal magnitude that is less than a magnitude of an output signal on the power-supply output node.

For example, where a power supply has a non-zero residual output signal (e.g., output voltage) on its output node after the power supply is deactivated, a power-supply controller can use such a technique to reduce or eliminate a drop in the residual output signal caused by, or that would be caused by, a restarting of the power supply. The power-supply controller reduces or eliminates the drop in the residual output voltage by isolating (e.g., tri-stating) the output node, analyzing the duty cycle of the power-supply pulse-width-modulated switching signal to determine the level of the output signal that the duty cycle would support if the output node were driven (i.e., not isolated), and driving the output node only after the output-signal level that the duty cycle would support is equal to, or greater than, the residual output signal. Furthermore, such a technique may not require the power-supply controller to have an additional pin to receive the output signal for the purpose of comparing it to the output-signal level that the duty cycle would support.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a buck-converter power supply, and of a load powered by the buck converter.

FIG. 2 is a diagram of one of the drive circuits of FIG. 1.

FIG. 3 is a plot of the supply output voltage generated by the buck converter power supply of FIG. 1 on restart of the buck converter.

FIG. 4 is a diagram of a buck converter power supply, and of a load powered by the buck converter, according to an embodiment.

FIG. 5 is a plot of a pulse-width-modulated (PWM) drive signal generated by the PWM circuit of FIG. 4, according to an embodiment.

FIG. 6 is a diagram of the signal-drop-reducing circuit of FIG. 4, according to an embodiment.

FIG. 7 is a plot of a filtered PWM drive signal generated by the signal-drop-reducing circuit of FIG. 6, and of the supply output voltage generated by the buck converter of FIG. 4, according to an embodiment.

FIG. 8 is a diagram of a portion of the signal-drop-reducing circuit of FIG. 6, according to an embodiment.

FIG. 9 is a plot of the filtered PWM drive signal generated by the signal-drop-reducing circuit of FIG. 6, and of the supply output voltage generated by the buck converter of FIG. 4, according to another embodiment.

FIG. 10 is a diagram of a circuit that can be included with, or as part of, the signal-drop-reducing circuits of FIGS. 6 and 8 to further reduce, or to eliminate, a drop in the supply output voltage of the buck converter of FIG. 4, according to an embodiment.

FIG. 11 is a diagram of a buck converter power supply operable in a discontinuous frequency mode (DFM), and of a load powered by the buck converter, according to an embodiment

FIG. 12 is a diagram of a boost converter power supply, and of a load powered by the boost converter, according to an embodiment.

FIG. 13 is a diagram of a buck-boost converter power supply, and of a load powered by the buck-boost converter, according to an embodiment.

FIG. 14 is a diagram of a single-ended-primary-inductor converter (SEPIC) power supply, and of a load powered by the SEPIC, according to an embodiment.

FIG. 15 is a diagram of a OUK converter power supply, and of a load powered by the OUK converter, according to an embodiment.

FIG. 16 is a diagram of a system that incorporates at least one of the power supplies of FIGS. 4 and 11-15, according to an embodiment.

DETAILED DESCRIPTION

FIG. 1 is a diagram of a power supply, here a buck converter 10, and a load 12 powered by the buck converter. As described below, a problem with the buck converter 10 is that when it is started or restarted with a non-zero residual output voltage V_(out), the buck converter may cause V_(out) to drop before V_(out) increases to its regulated voltage level (e.g., 1.1 Volts (V) or 3.3 V). Unfortunately, such a drop in V_(out) is undesirable in some applications.

The buck converter 10 includes a power-supply controller chip 14, one or more phase inductors 16 ₁-16 _(n), a feedback circuit 18, an output filter capacitor 20, an input node 22 configured to receive an input voltage V_(in), an output node 24 configured to provide the output voltage V_(out), a reference node 26 configured to receive a reference voltage (here ground), and phase nodes 28 ₁-28 _(n).

The power-supply controller chip 14 includes a soft-start circuit 30, a soft-start capacitor 32, an error amplifier 34, a controller 36, one or more switching-stage drive circuits 38 ₁-38 _(n), and one or more phase switching stages 40 ₁-40 _(n). Each switching stage 40 ₁-40 _(n) and corresponding phase inductor 16 ₁-16 _(n) forms a respective switching phase circuit 42 ₁-42 _(n). And although described as being part of the power-supply controller chip 14, the switching stages 40 ₁-40 _(n) can be separate from the controller chip.

The feedback circuit 18, which is optional, includes a resistor voltage divider 44 with resistors R₁ and R₂, and is configured to generate a feedback signal V_(FB) according to the following equation:

$\begin{matrix} {V_{FB} = {\frac{R_{2}}{R_{1} + R_{2}} \cdot V_{out}}} & (1) \end{matrix}$

If the feedback circuit 18 is omitted from the buck converter 10, then V_(FB)=V_(out).

The soft-start circuit 30 and capacitor 32 function together to generate a soft-start reference voltage, V_(soft-start), during a start or a restart of the buck converter 10. As discussed below, V_(soft-start) reduces the magnitude of, or eliminates, a transient voltage that the buck converter 10 superimposes on V_(out) when V_(out) is less than its regulated value. The soft-start circuit 30 generates, and provides to the capacitor 32, a substantially constant current I_(soft-start), which causes the capacitor to generate V_(soft-start) as a substantially linearly increasing, i.e., ramping, voltage.

The error amplifier 34 generates an error signal V_(error) in response to a difference between the actual value of V_(out) and V_(out)'s desired regulated value. During normal operation of the buck converter 10 in a continuous conduction mode (CCM) during which a current is always flowing through the inductor(s) 16 ₁-16 _(n) to the output node 24, the error amplifier 34 is configured to compare the feedback signal V_(FB) to a reference voltage (e.g., a bandgap reference voltage) V_(ref), and during a start or restart of the buck converter, the error amplifier is configured to compare V_(FB) to V_(soft-start). The larger the magnitude of the difference between V_(out) and either V_(FB) or V_(ref), the larger the magnitude of V_(error); conversely, the smaller the magnitude of the difference between V_(out) and either V_(FB) or V_(ref), the smaller the magnitude of V_(error). Although not shown in FIG. 1, the error amplifier 34 includes external or internal compensation, typically in the form of an RC network coupled to at least the inverting input node and the output node of a high-gain amplifier that forms part of the error amplifier. The compensation is suitable for the type of the power supply (the buck converter 10 in FIG. 1), the type of load, and for other parameters of the power supply, such as, but not limited to, response time, power output, current output, stability margin, clock frequency, and control type (e.g., PWM, constant on time, and current-mode control). Examples of suitable error-amplifier compensation include Types I, II, and III compensation. Because such compensation can be conventional, it is not described in detail herein.

The controller 36 includes a pulse-width-modulation (PWM) circuit 46, which generates, from a master clock, i.e., from a master PWM signal, a respective pulse-width-modulated (PWM) drive signal Drive₁-Drive_(n) for each of the switching phase circuits 42 ₁-42 _(n) in response to V_(error). Each signal Drive has the same duty cycle D, but has a different phase to reduce the amplitude of the ripple voltage on V_(out) in a conventional manner; typically, the phase difference between each pair of signals Drive is 360°/n. The duty cycle D is related to V_(error), and, therefore, is related to the difference between V_(FB) and either V_(ref) or V_(soft) _(_) _(start). For example, the more positive V_(error), the lower is V_(FB) compared to V_(ref) or V_(soft) _(_) _(start) (and thus the lower V_(out) is compared to its desired regulated value) and thus the larger the duty cycle D, so as to increase V_(out) toward its regulated value. Conversely, the less positive V_(error), the higher is V_(out) compared to V_(ref) or V_(soft) _(_) _(start) (and thus the higher V_(out) is compared to its regulated value), and thus the smaller the duty cycle D, so as to decrease V_(out) toward its regulated value.

FIG. 2 is a schematic diagram of the switching-stage drive circuit 38 ₁ of FIG. 1 according to an embodiment, it being understood that the other drive circuits 38 ₂-38 _(n) are similar.

Referring to FIGS. 1-2, each switching-stage drive circuit 38 ₁-38 _(n) converts the respective drive signal Drive₁-Drive_(n) into a pair of signals Switch_(High) and Switch_(Low) for driving the respective switching stages 40 ₁-40 _(n). For example, referring to FIG. 2, the switching-stage drive circuit 38 ₁ includes a non-inverting buffer 48 ₁, which generates Switch_(High1) equal to Drive₁, and includes an inverting buffer 50 ₁, which generates Switch_(Low1) equal to Drive1 . Therefore, in an embodiment, Switch_(High1)=Switch_(Low1) , although in another embodiment, the switching-stage drive circuit 38 ₁ is configured to generate Switch_(High1) and Switch_(Low1) so that they are both inactive (e.g., Logic 0=0 V) for a period of time to prevent a crowbar current from flowing in the switching stage 40 ₁. Furthermore, both of the buffers 48 ₁ and 50 ₁ are configured to generate an inactive voltage level, e.g., a logic 0, in response to a signal Enable having a level of logic 0, to deactivate the switching stage 40 ₁ as further discussed below. In an embodiment, the other drive circuits 38 ₂-38 _(n) have the same structure, and are configured to operate in the same manner, as the switching-stage drive circuit 38 ₁.

Referring again to FIG. 1, each switching stage 40 ₁-40 _(n) has a respective high-side switch, here a respective high-side transistor 52, e.g. NMOS transistor, and has a respective low-side switch, here a respective low-side transistor 54, e.g. NMOS transistor. In response to Switch_(High)=logic 1 (e.g., logic 1=the voltage at the respective phase node 28 (the bootstrapped voltage)+3.3 V) and Switch_(Low)=logic 0 (e.g., logic 0=0 V), a linearly increasing current flows from the input node 22, through the active high-side transistor 52 and the corresponding inductor 16, to the output node 24; because the low-side transistor 54 is inactive, negligible or no current flows through the low-side transistor. Conversely, in response to Switch_(High)=logic 0 and Switch_(Low)=logic 1, a linearly decreasing current flows from the reference node 26 (here ground), through the active low-side transistor 54 and the corresponding inductor 16, to the output node 24; because the high-side transistor 52 is inactive, negligible or no current flows through the high-side transistor. And when Switch_(High)=Switch_(Low)=logic 0, both transistors 52 and 54 are inactive such that the switching stage 40 is tri-stated, i.e., the corresponding phase node 28 presents a high impedance to the inductor 16, output filter capacitor 20, and load 12 such that negligible or no current flows from the output node 24 into the phase node.

Referring to FIGS. 1 and 2 normal operation of the buck converter 10 in CCM mode is described, according to an embodiment.

The error amplifier 34 generates V_(error) in response to the difference between V_(FB) and V_(ref); because V_(FB) is linearly related to V_(out) and V_(ref) is linearly related to the regulated level of V_(out), the error amplifier effectively generates V_(error) in response to the difference between V_(out) and its regulated level.

In response to V_(error), the PWM circuit 46 of the controller 36 generates the PWM signals Drive₁-Drive, each having a duty cycle D that causes the average value of V_(out) to move toward its regulated value, or to remain at its present value if the average of V_(out) is equal to its regulated value. The PWM circuit 46 is configured to generate Drive₁-Drive_(n) in response to a master clock signal, which can be the same as one of Drive₁-Drive_(n) (e.g., Drive₁)

The switching-stage drive circuits 38 ₁-38 _(n) convert the respective signals Drive₁-Drive_(n) into respective switching signals Switch_(High1)-Switch_(High) _(_) _(n) and Switch_(Low) _(_) ₁-Switch_(Low) _(_) _(n) for driving the high-side transistors 52 and low-side transistors 54 with the duty cycle D.

And the switched transistors 52 and 54 cause respective phase currents to flow from the input node (Vin) 22, through the inductors 16, and to the output node 24 for powering the load 12.

Still referring to FIGS. 1 and 2, operation of the buck converter 10 during a start-up mode or restart mode, when the buck converter is being activated or reactivated after a period of inactivity, is described, according to an embodiment.

The soft-start circuit 30 and capacitor 32 generate the reference voltage V_(soft) _(_) _(start), which starts at zero and linearly ramps up to V_(ref) over a period of time that is selected based on, e.g., the design of the buck converter 10 and the load 12. So, the buck converter 10 operates in a manner similar to that described above for the normal operation of the buck converter, except that the error amplifier 34 uses V_(soft) _(_) _(start) as the reference voltage until V_(soft) _(_) _(start) equals V_(ref), at which time the buck converter deactivates the soft-start circuit 30 and the error amplifier transitions to using V_(ref) as a reference instead of V_(soft) _(_) _(start). After the error amplifier 34 transitions to using V_(ref) as a reference, the buck converter 10 enters its normal mode of operation as described above.

A reason for the buck converter 10 using V_(soft) _(_) _(start), instead of V_(ref), as the reference voltage during start up is as follows. Upon starting the buck converter 10, V_(out) may be significantly lower than its desired regulated level. Therefore, V_(FB) may be significantly lower than V_(ref), and, in response to this significant difference, the error amplifier 34 would tend to generate V_(error) having a large positive value. This large, positive V_(error) would cause the PWM circuit 46 to generate the signals Drive with a large duty cycle D, which would cause the transistors 52 and 54 to switch in such a way as to generate large phase currents into the output node 24. Because, for loop stability and to filter out noise, the response time of the feedback loop that includes the error amplifier 34 is relatively slow, after V_(out) were to become equal to its regulated value, the large phase currents might cause a significant transient overshoot in V_(out) during the time (sometimes called the transient-response time) that the feedback loop requires to reduce the phase currents to values that stabilize V_(out) at its regulated value. But because V_(soft) _(_) _(start) ramps from 0 V at a rate that is comparable to the response time of the feedback loop, the difference between V_(FB) and V_(soft) _(_) _(start) typically is not so large as to superimpose an excessive transient voltage on V_(out).

In another embodiment, the error amplifier 34 is configured so that it generates V_(error)=0 if V_(soft) _(_) _(start)<V_(FB) such that the switching stages 40 are disabled, and enables the switching stages 40, and begins to generate a non-zero value for D, only when V_(soft) _(_) _(start)≧V_(FB). Therefore, the error amplifier 34 configured in this manner may further reduce the transient voltage that the buck converter 10 superimposes on V_(out) during start or restart.

But even when the error amplifier 34 uses V_(soft) _(_) _(start) as a reference during startup, the feedback loop may still superimpose a significant transient voltage on V_(out) if there is a significant residual V_(out) on the output node 24 when the buck converter 10 starts or restarts.

A reason that such a residual V_(out) may exist is discussed. During normal operation of the buck converter 10, the output filter capacitor 20 is charged to the regulated level of V_(out). To adequately filter V_(out), the output filter capacitor 20 is relatively large, and has a value ranging from tens of microfarads to hundreds of microfarads depending on the application. When the buck converter 10 is deactivated, e.g., during a low-power mode, such as a sleep mode, of the load 12, the drive circuits 38 deactivate the switching stages 40 such that the phase nodes 28 present high impedance to the output filter capacitor 20 as described above. Therefore, because, during a low-power mode, the load 12 also presents high impedance to the output filter capacitor 20, the only paths through which a capacitor-discharge current can flow are leakage paths that have relatively high impedances. Therefore, it may take a relatively long time, e.g., seconds, tens of seconds, or even minutes, for the voltage across the output filter capacitor 20 to decay to 0 V.

Because V_(soft) _(_) _(start) ramps up from 0 V, if the residual voltage V_(out) is significant, then, upon starting the buck converter 10, V_(out) may be significantly higher than V_(soft) _(_) _(start). Therefore, V_(FB) may be significantly higher than V_(soft) _(_) _(start), and, in response to this significant difference, the error amplifier 34 would tend to generate V_(error) at its lowest value. This lowest value of V_(error) would cause the PWM circuit 46 to generate the signals Drive with a small duty cycle D, which would cause the transistors 52 and 54 to switch in such a way as to cause V_(out) to fall to a lower value; that is, the small duty cycle D could cause V_(out) to discharge, and to thus fall, from its previous residual value. In more detail, because the response time of the voltage-regulating feedback loop is relatively slow as discussed above, even if the buck converter 10 is configured to disable the switching stages 40 until V_(soft) _(_) _(start)≧V_(FB), the slow response time of the regulation loop may delay the increase in V_(error), and, therefore, may cause D, and thus the output voltage corresponding to D, to increase slowly for a period of time during which V_(out) may experience a voltage drop.

FIG. 3 is a plot of V_(out) on the output node 24 of the buck-converter 10 of FIG. 1, according to an example where V_(out) experiences a significant reduction, i.e., drop, from its residual value V_(out) _(_) _(residual) during a restart period of the buck converter 10.

Referring to FIGS. 1 and 3, during a period t₀-t₁, V_(out) has a non-zero residual value V_(out) _(_) _(residual) (the period t₀-t₁ is assumed to be short enough relative to the discharge time constant of the output filter capacitor 20 so that V_(out) _(_) _(residual) can be assumed to be constant during this time period). At time t₁, the buck converter 10 starts or restarts, e.g., because the load 12 is “awakening” from a “sleep” or other idle mode. If the buck converter 10 starts or restarts by commencing a soft-start cycle, because V_(soft) _(_) _(start) is 0 V, the difference between V_(FB) and V_(soft) _(_) _(start) at time t₁ is so large, and the response time of the feedback loop so slow, that the buck converter 10 causes a significant transient drop in V_(out), from V_(out) _(_) _(residual) to V_(out) _(_) _(drop), before the feedback loop can recover and begin to increase V_(out) toward its regulated value. The difference between V_(out) _(_) _(residual) and V_(out) _(_) _(drop) is typically in the range from a few tens of millivolts (mV) to hundreds of millivolts, or even to 1 V or more. And, as stated above, even if the error amplifier 34 is configured to generate V_(error)=0 in response to a large difference between V_(FB) and V_(soft) _(_) _(start), and even if the buck converter 10 is configured to disable the switching stages 40 until V_(soft) _(_) _(start)≧V_(FB), the slow response time of the regulation loop still can cause a drop in V_(out), although perhaps not as severe a drop as described above.

Still referring to FIGS. 1 and 3, in at least some applications, such a drop in V_(out) from its residual value V_(out) _(_) _(residual) to a drop value V_(out) _(_) _(drop) is undesirable. For example, such drop in V_(out) represents a loss of energy such that if the load 12 is battery powered, this voltage drop may shorten the amount of time that the load can operate between battery recharges, particularly if the load frequently enters and exits one or more low-power states. Furthermore, if V_(out) _(_) _(residual) is higher than a start-up threshold supply voltage when the load 12 exits a low-power state, then the load may not need to reinitialize, e.g., configuration and other memory registers, and, therefore, this may shorten the time, and reduce the amount of energy, that it takes the load to exit the low-power state; such a situation may arise when the load is in the low-power state for a relatively short period of time, which is common in today's “green” electronic devices. But if the buck converter 10 causes V_(out) _(_) _(residual) to drop below the start-up threshold supply voltage of the load 12, then the load may require a significantly longer time, and significantly more energy, to “wake up.” That is, if the buck converter 10 were not to cause a significant drop in V_(out) _(_) _(residual), then this would increase the chances that upon “awakening,” the load 12 could execute a shorter and less-energy-consuming “wake-up” routine as compared to the “wake-up” routine that the load executes when V_(out) is below the start-up threshold supply voltage. Moreover, a significant drop in V_(out) _(_) _(residual) also can affect analog circuits adversely. For example, a phase-lock loop may be forced to reacquire lock during a reacquisition time if its power-supply voltage drops below a minimum threshold.

FIG. 4 is a diagram of a buck converter 60, which includes a signal-drop-reducing circuit 62 for reducing or eliminating a drop in V_(out) _(_) _(residual) on start or restart of the buck converter, according to an embodiment. In FIG. 4, components common to the buck converter 10 of FIG. 1 may be labeled with the same reference numbers, and, for brevity, are not described again.

The signal-drop-reducing circuit 62 reduces or eliminates a drop in V_(out) _(_) _(residual) by generating a signal Enable having a value (e.g., logic 0) that disables the switching stages 40, and that thus causes the phase nodes 28 to present a high impedance to the output filter capacitor 20, until the voltage corresponding to the duty cycle D of the PWM signals Drive₁-Drive_(n) is equal to or greater than V_(out) _(_) _(residual). Therefore, by the time that the signal-drop-reducing circuit 62 enables the switching stages 40, the duty cycle D is large enough to ensure that the high-side transistors 52 are active long enough compared to the low-side transistors 54 so that little or no output-voltage-lowering reverse currents flow through the inductors 16. Consequently, the buck converter 60 causes little or no drop in V_(out) _(_) _(residual) such that when the signal-drop-reducing circuit 62 finally activates the switching stages 40, V_(out) rises from V_(out) _(_) _(residual) to its regulated value with little or no transient drop.

The signal-drop-reducing circuit 62 functions as described above by taking advantage of the known relationship between V_(in), V_(out), and D to determine, in response to D, a voltage that corresponds to D and when this corresponding voltage equals, or is greater than, V_(out) _(_) _(residual).

FIG. 5 is a plot of the drive signal Drive₁ of FIG. 4, according to an embodiment. It is understood, however, that a similar analysis applies to the drive signals Drive₂-Drive_(n), and to a master clock or PWM signal from which the signals Drive may be generated.

Referring to FIGS. 4 and 5, during normal, CCM operation of the buck converter 60, Drive₁ equals K·V_(in) (K is a constant) during a time T₁ of a PWM switching period T, Drive₁ equals 0 V during a time T₂ of the PWM switching period T, T₁+T₂=T, the high-side transistor 52 ₁ is active during T₁ and is inactive during T₂, and the low-side transistor 54 ₁ is inactive during T₁ and is active during T₂. The relationship between D, V_(in), and V_(out), for an ideal buck converter, is given by the following equation:

$\begin{matrix} {V_{out} = {{V_{in} \cdot \frac{T_{1}}{T}} = {V_{in} \cdot D}}} & (2) \end{matrix}$

where

${D = \frac{T_{1}}{T}},$

and UUM operation means that there are always currents flowing through the inductors 16 into the output node 24 during the entire PWM period T. And the constant K is a scale factor introduced because the Drive₁ signal is typically a digital logic level (such as 0 V to 3.3 V), and V_(in) could be a different level (such as 12 V). It is understood that equation (2) describes the behavior of an ideal buck converter with 100% efficiency. However, it is typically a good-enough approximation for high-efficiency buck converters, for example buck converters with efficiencies of 90% or higher. In the remainder of this disclosure, ideal equations will be used with the understanding that they are good-enough approximations of the behavior of high-efficiency converters.

Although during start or restart of the buck converter 60 equation (2) does not hold because the phase nodes 28 present high impedances to the output node 24, one can still determine the equivalent output voltage, V_(out) _(_) _(eq), that the duty cycle D would generate if the switching stages 40 here active, according to the following equation for an ideal buck converter:

V _(out) _(_) _(eq) =V _(in) ·D   (3)

That is, V_(out) _(_) _(eq) is the voltage to which D corresponds when the switching stages 40 are disabled, whereas V_(out) is the voltage to which D corresponds, and which D causes the buck converter 60 to generate on the output node 24, when the switching stages are enabled.

FIG. 6 is a diagram of the signal-drop-reducing circuit 62 of FIG. 4, according to an embodiment.

The signal-drop-reducing circuit 62 includes an amplifier 70, a first low-pass filter 72, a second low-pass filter 74, a comparator 76, a latch 78, and AND gates 80 ₁-80 _(n).

The amplifier 70 multiplies the drive signal Drive_(l) by 1/K, and the low-pass filter 72 effectively takes the average of Drive₁/K to generate, at least approximately, V_(out) _(_) _(eq) per equation (4) below. As described below in conjunction with FIG. 7, depending on its frequency response, the filter 72 may generate an approximate version of V_(out) _(_) _(eq) with a small ripple voltage.

Referring again to FIG. 5, one can see that V_(out) _(_) _(eq) equals the time average of

$\frac{{Drive}_{1}}{K}$

according to the following equation, for an ideal buck converter:

$\begin{matrix} {V_{{out}\_ {eq}} = {\frac{\left( \frac{{Drive}_{1}}{K} \right) \cdot T_{1}}{T} = {\frac{\left( \frac{KVin}{K} \right) \cdot T_{1}}{T} = {\frac{{Vin} \cdot T_{1}}{T} = {{Vin} \cdot D}}}}} & (4) \end{matrix}$

The low-pass filter 74, which is optional in this embodiment, generates V_(out) _(_) _(residual) _(_) _(filtered) by effectively taking the average of V_(out) _(_) _(residual) while the buck converter 60 (FIG. 4) is starting or restarting.

The comparator 76 compares V_(out) _(_) _(eq) from the first low-pass filter 72 and V_(out) _(_) _(residual) _(_) _(filtered) from the second low-pass filter 74 (V_(out) _(_) _(residual) _(_) _(filtered)=V_(out) _(_) _(residual) if the second low-pass filter is omitted from the circuit 62), and generates the signal Enable having a disable value (e.g., logic 0) if V_(out) _(_) _(eq)≦V_(out) _(_) _(residual) _(_) _(filtered) and having an enable value (e.g., logic 1) if V_(out) _(_) _(eq)>V_(out) _(_) _(residual) _(_) _(filtered). That is, the comparator 76 generates Enable having an enable value only if the voltage corresponding to the duty cycle D of Drive₁ is greater than V_(out) _(_) _(residual) _(_) _(filtered).

The latch 78 latches Enable upon the transition of Enable from its disable value (e.g., logic 0) to its enable value (e.g., logic 1) to generate Enable_L, which is the latched version of Enable.

And the AND gates 80 ₁-80 _(n), which are optional, generate gated versions Drive₁ _(_) _(g)-Drive_(n) _(_) _(g) of the drive signals Drive₁-Drive_(n), respectively, in response to Enable_L having an enable value; when Enable_L has a disable value, the AND gates generate Drive_(1-g)=Drive₂ _(_) _(g)= . . . =Drive_(n) _(_) _(g)=0.

Referring to FIGS. 4 and 6, although the power-supply controller chip 14 has no pin that is directly connected to the output node 24, the low-pass filter 74 can receive V_(out) _(_) _(residual) via one of the phase nodes 28 to avoid the need to add an additional pin to the controller. Because the signal-drop-reducing circuit 62 operates while the phase nodes 28 present high impedance to the output node 24, sometime after the switching stages 40 are disabled, negligible or no current flows through the inductors 16 such that V_(out) _(_) _(residual) also appears on the phase nodes. And even though V_(out) does not appear on the phase nodes 28 after the signal-drop-reducing circuit 62 activates the switching stages 40, the signal-drop-reducing circuit need not receive V_(out) _(_) _(residual) or V_(out) once it generates Enable_L having an enable level since the latch 78 latches the enable level of Enable_L.

FIG. 7 is a plot of V_(out) _(_) _(residual) on the output node 24 of FIG. 4 and of V_(out) _(_) _(eq) of FIG. 6, according to an embodiment. In this example, V_(out) _(_) _(eq) has a ripple voltage, V_(rip), superimposed on it, where the amplitude of V_(rip) is related to the frequency response of the low-pass filter 72 of FIG. 6.

Referring to FIGS. 4-7, operation of the buck converter 60 during start and restart is described, according to an embodiment in which the error amplifier 34 is configured to generate V_(error)=0 at least until V_(soft) _(_) _(start)=V_(FB). Furthermore, for purposes of example, V_(out) _(_) _(residual) is assumed to equal 3 V and to be constant until the signal-drop-reducing circuit 62 enables the switching stages 40.

First, in response to a start/restart signal (not shown in FIGS. 4-7) from the load 12, at a time t₀ the power-supply controller chip 14 activates the soft-start circuit 30, which commences ramping up V_(soft) _(_) _(start) from 0 V (V_(soft) _(_) _(start) not shown in FIGS. 4-7).

Next, during a time period from t₀ to a time t₁, V_(soft) _(_) _(start)<V_(FB) such that V_(error)=Drive₁=0 (i.e., the duty cycle D of Drive₁ equals zero). Because Drive₁, and thus V_(out) _(_) _(eq), are less than V_(out) _(_) _(residual), the comparator 76 generates the signal Enable having a disable level (e.g., logic 0) such that the switching stages 40 are disabled and the phase nodes 28 present high impedances to the output node 24 of the buck converter 10.

Then, at t₁, V_(soft) _(_) _(start) exceeds V_(FB) such that V_(error) begins to increase from a starting voltage of 0 V. In response to V_(error) increasing from 0 V, the PWM circuit 46 generates Drive₁ having a minimum, nonzero, duty cycle 0<D<<1. That is, D effectively “jumps” from zero to a minimum nonzero value, which depends on, e.g., the design and configuration of the PWM circuit. Typically, the reason that the PWM circuit 46 generates Drive₁ having a minimum nonzero value for D is because due to circuit-timing limitations, the PWM circuit cannot generate Drive₁ having a duty cycle D less than this minimum nonzero value, at least not in a reliable manner. In response to Drive₁ having the minimum duty cycle, the low-pass filter 72 generates V_(out) _(_) _(eq) having a minimum amplitude that corresponds to the minimum duty cycle D.

Next, during a time period from t₁ to a time t₂, although V_(soft) _(_) _(start) continues to increase, V_(out) _(_) _(residual), and thus V_(FB), remain at constant values because the signal-drop-reducing circuit 62 is still disabling the switching stages 40. And although the difference between V_(soft) _(_) _(start) and V_(FB) is increasing, as is V_(error), the increasing V_(error) is still not high enough to cause the PWM circuit 46 to increase the duty cycle D of the signal Drive₁ above its minimum value.

Then, at t₂, the difference between the still-increasing V_(soft) _(_) _(start) and the still-constant V_(FB) is large enough to increase V_(error) to a level that is sufficient to cause the PWM circuit 46 to increase the duty cycle D of the signal Drive₁ above the minimum duty-cycle value. But because V_(out) _(_) _(eq) is still less than V_(out) _(_) _(residual), the comparator 76 continues to disable the switching stages 40 by generating the signal Enable having a disable level.

Next, during a time period from t₂ to a time t₃, V_(soft) _(_) _(start) continues to increase, V_(FB) continues to remain at a constant value, and V_(error), D, and V_(out) _(_) _(eq) continue to increase. Yet V_(out) _(_) _(eq) is still less than V_(out) _(_) _(residual) such that the comparator 76 continues to disable the switching stages 40 by generating Enable having a disable level.

Then, at t₃, V_(error), and thus D, are high enough such that V_(out) _(_) _(eq) exceeds V_(out) _(_) _(residual). In response to V_(out) _(_) _(eq) exceeding V_(out) _(_) _(residual), the comparator 76 generates Enable having an enable level, in response to which the latch 78 latches Enable and generates Enable_L also having an enable level. And in response to Enable_L having an enable level, the AND gates 80 ₁-80 _(n) generate Drive₁ _(_) _(g)-Drive_(n) _(_) _(g) respectively equal to Drive₁-Drive_(n), and enable the switching stages 40. Because the signal-drop-reducing circuit 62 enables the switching stages 40 only after V_(out) _(_) _(eq) exceeds V_(out) _(_) _(residual), the buck converter 62 causes little or no voltage drop in V_(out) when the switching stages are activated (for purposes of naming convention, the name of the voltage on the output node 24 of the buck-converter 10 changes from V_(out) _(_) _(residual) to V_(out) upon the enabling of the switching stages 40, and transitions from V_(out) to V_(out) _(_) _(residual) upon disabling of the switching stages).

And even though at a time t₄ V_(out) _(_) _(eq) falls below V_(out) (the voltage on the output node 24 is now named V_(out) because the switching stages 40 are enabled), the switching stages remain enabled because the latch 78 already has latched Enable_L having an enable level.

After t₃, V_(soft) _(_) _(start) continues to increase until it equals V_(ref), at which point the error amplifier 34 begins to compare V_(FB) to V_(ref) instead of to V_(soft-start). Furthermore, V_(FB), V_(error), and the duty cycle D of Drive₁ _(_) _(g)-Drive_(n) _(_) _(g) increase until V_(out) equals its regulated voltage level.

Referring to FIGS. 4-7, alternate embodiments of the buck converter 60 are contemplated. For example, although described as a multiphase buck converter, the buck converter 60 may be a single-phase buck converter that otherwise operates in a manner similar to that described above. Furthermore, although the signal-drop-reducing circuit 62 is described as generating V_(out) _(_) _(eq) from the signal Drive₁, the signal-drop-reducing circuit can generate V_(out) _(_) _(eq) from any of the other signals Drive₂-Drive_(n), or from a master PWM signal from which Drive₁-Drive_(n) are generated. Moreover, the inductors 16 of each phase may be magnetically uncoupled from one another, or two or more of the phase inductors may be magnetically coupled to one another. In addition, instead of including two transistors 52 and 54, e.g. NMOS transistors, as switches, each switching stage 40 may include a PMOS transistor for the high-side switch, for the low-side switch, or both the high-side and low-side switches. Furthermore, as described above in conjunction with FIG. 1, in an embodiment, the error amplifier 34 includes suitable internal or external compensation that causes the buck converter 60 to operate according to the above-described embodiment.

FIG. 8 is a diagram of the signal-drop-reducing circuit 62 of FIG. 4 according to another embodiment in which the signals V_(out) _(_) _(residual) and Drive₁ are scaled, or level shifted, so that their voltage levels are compatible with the voltage levels within the power-supply controller chip 14. Furthermore, components common to FIGS. 6 and 8 are labeled with the same reference numbers.

In addition to the low-pass filters 72 and 74, comparator 76, latch 78, and AND gates 80 ₁-80 _(n), the signal-drop-reducing circuit 62 includes two level shifters 90 and 92 for scaling V_(out) _(_) _(residual) and Drive₁ to be compatible with the supply voltage V_(dd) used internal to the power-supply controller chip 14 (FIG. 4).

The level shifter 90 includes resistors R₃ and R₄ arranged as a voltage divider such that the signal V_(out) _(—residual) _(—shifted) that the level shifter provides to the low-pass filter 74 is giving by the following equation:

$\begin{matrix} {V_{{{out}\_ {residual}}{\_ {shifted}}} = {\frac{R_{4}}{R_{3} + R_{4}} \cdot V_{{out}\_ {residua}l}}} & (5) \end{matrix}$

where

$\frac{R_{4}}{R_{3} + R_{4}} = \frac{1}{6}$

in an example application. In this example, a factor of ⅙ is sufficient for the amplitudes of the signals input to the comparator 76 to be less than V_(dd).

The level shifter 92 includes resistors R₅ and R₆ arranged as a voltage divider, and a logic buffer 94, such that the logic-1 value of the signal Drive₁ _(_) _(shifted) that the level shifter provides to the low-pass filter 72 is giving by the following equation:

$\begin{matrix} {{Drive}_{1{\_ {shifted}}{\_ logic}\_ 1} = {V_{in} \cdot \frac{R_{6}}{R_{5} + R_{6}}}} & (6) \end{matrix}$

where

$\frac{R_{6}}{R_{5} + R_{6}} = \frac{1}{6}$

in an example application. In more detail, the resistors R₅ and R₆ provide V_(in)/6 to the supply node of the buffer 96. Because Drive₁ alternates between logic 0 and logic 1, R₅ and R₆ effectively set logic 1 for Drive₁ equal to V_(in)/6 (logic 0=ground=0 V in this example). And by effectively setting logic 1 for Drive₁ equal to V_(in)/6, the buffer 96 eliminates the need for the 1/K amplifier of FIG. 6.

Depending on the design and configuration of the signal-drop-reducing circuit 62, the signals Drive₁-Drive_(n) may or may not be level shifted in a similar manner before being input to the AND gates 80 ₁-80 _(n), respectively. If the signals Drive₁-Drive_(n) are level shifted before being input to the AND gates 80 ₁-80 _(n), then another level shift occurs at some point between the AND gates 80 ₁-80 _(n) and the switching stages 40 ₁-40 _(n) (FIG. 4) so that the signals Switch_(High) and Switch_(Low) are compatible with V_(in) and V_(out), which are, respectively, the supply voltage to which the switching stages are coupled and the output voltage generated by the buck converter 60 (FIG. 4).

Furthermore, because the output of the low-pass filter 72 is coupled to the inverting input of the comparator 76 and the output of the low-pass filter 74 is coupled to the non-inverting input node of the comparator, an inverter 96 is coupled between the output of the comparator and the latch 78 so that Enable and Enable_L have the same polarities as described above in conjunction with FIG. 6.

Still referring to FIG. 8, alternate embodiments of the signal-drop-reducing circuit 62 are contemplated. For example, the level shifters 90 and 92 may have other topologies. Moreover,

$\frac{R_{4}}{R_{3} + R_{4}}$

may not equal

$\frac{R_{6}}{R_{5} + R_{6}}.$

FIG. 9 is an expanded view of a portion of the plot V_(out) _(_) _(residual) and V_(out) _(_) _(eq) of FIG. 7, according to an embodiment.

Referring to FIG. 9, a potential problem with the signal-drop-reducing circuit 62 of FIGS. 4, 6, and 8 is described, according to an embodiment.

Ideally, the signal-drop-reducing circuit 62 of FIG. 4 enables the switching stages 40 (also of FIG. 4) in response to at least the average of V_(out) _(_) _(eq), V_(out) _(_) _(eq) _(_) _(avg), being equal to or greater than V_(out) _(_) _(residual) to prevent a voltage drop in V_(out).

But referring to FIG. 9, and as described above in conjunction with FIGS. 4-7, the comparator 76 transitions the signal Enable to an enable level at the time t₃ when the ripple component V_(rip) of V_(out) _(_) _(eq) exceeds V_(out) _(_) _(residual).

At the time t₃, V_(out) _(_) _(eq) _(_) _(avg) is still slightly less than V_(out) _(_) _(residual), and, therefore, the duty cycle D of Drive₁ may still be slightly less than the value needed to prevent a drop in V_(out) _(_) _(residual) upon enabling the switching stages 40 (FIG. 4).

Consequently, the signal-drop-reducing circuit 62 may still cause a slight drop, V_(drop), in V_(out) from V_(out) _(_) _(residual).

One technique for reducing or eliminating this slight V_(drop) is for the signal-drop-reducing circuit 62 to enable the switching stages 40 (FIG. 4) only at, or after, a time t₅ at which V_(out) _(_) _(eq) _(_) _(avg) equals V_(out) _(_) _(residual).

FIG. 10 is a diagram of a circuit 100, which, when included in the signal-drop-reducing circuit 62 of FIG. 6 or FIG. 8, causes the latch 78 to generate and to latch the signal Enable_L having an enable level only if the signal Enable attains an enable level while a master PWM clock signal PWM CLOCK is active (e.g., logic 1), and retains this enable level until the next active edge (e.g., rising edge) of PWM CLOCK.

The circuit 100 is effectively coupled between the comparator 76 and the latch 78 of FIG. 6, or between the inverter 96 and the latch 78 of FIG. 8, and includes an inverter 102, a NAND gate 104, an inverter 106, two NOR gates 108 and 110, and a D flip-flop 112. The inverter 102 and an input node of the NAND gate 104 receive the signal Enable from the comparator 76 (or inverter 96), and the other input node of the NAND gate receives PWM CLOCK. And the D flip-flop 112 has a D input (the D input is different from the duty cycle D) coupled to receive the output of the NOR gate 108, has a CLOCK input coupled to receive PWM CLOCK, and has a Q output that provides a signal Enable_Intermediate to the Latch 78.

PWM CLOCK is a master clock signal that has the same period T as the signal Drive₁ (and the other signals Drive₁-Drive_(r)) per FIG. 5, and, in an embodiment, that has a rising edge that coincides with the rising edge of Drive₁.

The circuit 100 operates as follows according to an embodiment in which the disable level of Enable is logic 0 and the enable level of Enable is logic 1.

Initially, Enable has a logic 0 disable level such that the NOR 108 generates a logic 0 output and the circuit 100 generates Enable_Intermediate having a logic 0 disable level.

But if Enable attains a logic 1 enable level while PWM CLOCK has a logic 1 level (i.e., during the positive portion of PWM CLOCK), and maintains this enable level until the next rising edge of PWM CLOCK, then the D flip-flop 112 generates Enable_Intermediate having a logic 1 enable level in response to the next rising edge of PWM CLOCK.

By operating in this manner, the circuit 100 causes Enable_Intermediate, and thus Enable_L (FIGS. 4 and 8) to have an enable level, and, therefore, enables the switching stages 40 (FIG. 4), at a time t₆ of FIG. 9 when, even at its lowest point, V_(out) _(_) _(eq) exceeds V_(out) _(_) _(residual). That is, at time t₆, not only is V_(out) _(_) _(eq) _(_) _(avg) greater than V_(out) _(_) _(residual), but V_(out) _(_) _(eq) is greater than V_(out) _(_) _(residual) even at the valleys of V_(out) _(_) _(eq).

Still referring to FIG. 10, alternate embodiments of the circuit 100 are contemplated. For example, the circuit 100 may have a logic topology other than that described. Furthermore, the combination of the circuit 100 and the comparator 76 of FIG. 6, or the combination of the circuit 100 and the comparator 76 and inverter 96 of FIG. 8, may be referred to as a comparator circuit. Or, the combination of the circuit 100 and the latch 78 of FIGS. 6 and 8 may be referred to as a latch circuit. Alternatively, if the circuit 100 is omitted, the combination of the comparator 76 and the latch 78 may be referred to as a comparator circuit or as a latch circuit. Moreover, the circuit 100 could also be eliminated if the bandwidth of the comparator 76 were low compared to the PWM frequency such that the comparator effectively acts as a low-pass filter that generates an enable signal only when V_(out) _(_) _(eq) _(_) _(av)>V_(out) _(_) _(residual)

Referring to FIGS. 6, 8, and 10, in an embodiment in which the signal-drop-reducing circuit 62 includes the circuit 100 of FIG. 10, the signal-drop-reducing circuit can be configured to further reduce the chances that V_(out) will experience a drop in voltage from V_(out) _(_) _(residual) upon enabling the switching stages 40 of FIG. 4. For example, the signal-drop-reducing circuit 62 can be configured to enable the switching stages 40 only when at least Drive₁ has a high (logic 1) level such that the high-side transistor 52 ₁ of the switching stage 40 ₁ will be the first of the two transistors in the stage 40 ₁ to turn on upon enabling of the switching stage 40 ₁.

If the switching stage 40 ₁ is enabled when Drive₁ has a logic 0 level, then the low-side transistor 54 ₁ will turn on and the high-side transistor 52 ₁ will remain off.

Because there is no current flowing in the inductor 16 ₁, turning on the low-side transistor 54 ₁ causes a reverse current to flow from the output node 24, through the inductor 16 ₁ and the low-side transistor 54 ₁, to ground.

Although this reverse current is likely to be short lived once the high-side transistor 52 ₁ turns on in response to Drive₁ transitioning to a high level, this reverse current could still cause a slight drop in V_(out).

Consequently, one can configure the signal-drop-reducing circuit 62 so that it does not enable the switching stage 40 ₁ until Drive₁ is at a high level such that upon the switching stage being enabled, the first transistor to turn on is the high-side transistor 52 ₁. Turning on the high-side transistor 52 ₁ first causes a forward current, not a reverse current, to flow in the inductor 16 ₁, and, therefore, prevents a drop in V_(out) that might otherwise occur if the low-side transistor 54 ₁ is turned on first.

Referring to FIGS. 6 and 8, to configure the signal-drop-reducing circuit 62 per the preceding paragraph, one can include delay or other synchronizing circuits (not shown in FIGS. 6 and 8) between the signal Drive₁ and the respective input node of the AND gate 80 ₁.

Operation of the so-configured signal-drop-reducing circuit 62 is now described, according to an embodiment.

When the signal-drop-reducing circuit 62 includes the circuit 100 of FIG. 10, Enable_Intermediate transitions to a logic 1 enable level on a rising edge of PWM CLOCK, which is coincident with the rising edge of Drive₁.

But the logic 1 enable level of Enable_Intermediate endures a delay through, e.g., the latch 78, and possibly other circuitry, before it arrives at the respective input node of the AND gate 80 ₁.

Therefore, by the time the logic 1 enable level arrives at the AND gate 80 ₁, Drive₁ may have already transitioned to a logic 0 low level (which means that it would turn on the low-side transistor 54 ₁ upon enabling of the switching stage 40 ₁).

But if Drive₁ is sufficiently delayed or otherwise synchronized, for example, by the same delay, or by approximately the same delay, that the logic 1 enable level of Enable_Intermediate endures between the D flip-flop 112 and the input of the AND gate 80 ₁, then this delayed or otherwise synchronized version of Drive₁ will have a logic 1 level when the switching stage 40 ₁ is enabled. That is, the delay or other synchronization of Drive₁ synchronizes, or approximately synchronizes, the rising edge of Enable_L with the rising edge of Drive₁ such that the high-side transistor 52 ₁ is the first transistor of the stage 40 ₁ turned on upon enabling of the stage, and is turned on for the entire, or almost entire, duration T₁ of Drive₁ (see FIG. 5).

Because the signals Drive₂-Drive_(n) are out of phase with Drive₁, the delays or other synchronizations for each of these other signals Drive may need to be different to make sure that the high-side transistors 52 ₂-52 _(n) are the first transistors turned on upon enabling the stages 40 ₂-40 _(n), respectively.

Alternatively, Drive₁ may be the only one of the signals Drive₁-Drive_(n) delayed or synchronized under the assumption that the forward current generated by the high-side transistor 52 ₁ through the inductor 16 ₁ is enough to offset any reverse currents generated by the low-side transistors 54 ₂-54 _(n) through the inductors 16 ₂-16 _(n), respectively.

FIG. 11 is a diagram of a buck converter 120, which is operable in both a CCM and in a Discontinuous Current Mode (DCM), which is described below, according to an embodiment. Components of the buck converter 120 common to the buck converters 10 and 60 of FIGS. 1 and 4, respectively, are labeled with the same reference numbers, and are not further described in conjunction with FIG. 11.

In addition to the components of the buck converter 60 of FIG. 4, the buck converter 120 includes comparators 122 and 124, an offset generator 126 configured to generate a constant (DC) offset voltage V_(off), and an RS flip-flop 128 configured to generate a signal PULSE SKIP.

The comparator 122 has an inverting input node coupled to receive V_(ref), a non-inverting input node coupled to receive a voltage equal to V_(FB)-V_(off), and an output node coupled to the set S input of the flip-flop 128.

The comparator 124 has an inverting input node coupled to receive V_(FB), a non-inverting input node coupled to receive V_(ref), and an output node coupled to the reset R input node of the flip-flop 128.

And the flip-flop 128 has a Q output node that provides PULSE SKIP to the controller 36. When the flip-flop 128 is reset, PULSE SKIP equals logic 0, in response to which the controller 36 restarts the buck converter 120. In contrast, when the flip-flop 128 is set, PULSE SKIP equals logic 1, in response to which the controller 36 deactivates the buck converter 120 at least by disabling the switching stages 40 ₁-40 _(n).

Typically, the buck converter 120 enters DCM during light-load conditions. The buck converter 120 may itself detect a light-load condition and put itself into DCM, or another circuit, such as the load 12, may put the buck converter into DCM.

During DCM, the buck converter 120 provides a current to the output node 24 periodically, instead of continuously, to increase efficiency.

The operation of the buck converter 120 in DCM is now described, according to an embodiment. In continuous conduction mode (CCM) and in start and restart into CCM, the buck converter 120 is configured to operate according to at least one of the embodiments described above in conjunction with FIGS. 4-10.

First, the buck converter 120 enters DCM from CCM by disabling the switching stages 40 ₁-40 _(n) such that the phase nodes 28 present high impedances to the output node 24.

Next, V_(out) is allowed to decay until V_(FB) is less than V_(ref) (the rate of decay is typically slow because the load is light and the filter capacitor has no low-impedance discharge paths as described above in conjunction with FIGS. 1-3).

In response to V_(FB)<V_(ref), the comparator 124 transitions its output from a logic 0 to a logic 1 to reset the flip-flop 128, which thus generates PULSE SKIP equal to logic 0.

In response to PULSE SKIP being equal to logic 0, the controller 36 causes the buck converter 120 to restart according to one of the embodiments described above in conjunction with FIGS. 4-10. That is, the controller 36 causes the buck converter 120 to restart in a manner that causes little, or no, voltage drop in V_(out) upon enabling the switching stages 40 ₁-40 _(n).

After the signal-drop-reducing circuit 62 enables the switching stages 40 ₁-40 _(n), the buck converter 120 causes V_(out) to increase.

The buck converter 120 continues to cause V_(out) to increase until V_(FB) exceeds V_(ref)+V_(off), at which time the comparator 122 transitions its output from logic 0 to logic 1 to set the flip-flop 128, which thus generates PULSE SKIP equal to logic 1.

In response to PULSE SKIP being equal to logic 1, the controller 36 disables the switching stages 40 ₁-40 _(n).

The buck converter 120 repeats the above procedure such that during DCM, V_(out) oscillates between

$V_{out} = {{{V_{ref} \cdot \frac{R_{1} + R_{2}}{R_{2}}}\mspace{14mu} {and}\mspace{14mu} V_{out}} = {\left( {V_{ref} + V_{off}} \right) \cdot \frac{R_{1} + R_{2}}{R_{2}}}}$

at a frequency that depends on the load 12.

Still referring to FIG. 11, alternate embodiments of the buck converter 120 are contemplated. For example, because, when the comparator 124 resets the flip-flop 128, V_(out) is only slightly below its regulated value, instead of using V_(soft) _(_) _(start) as a reference during the restart of the buck converter 120, the error amplifier 34 can be configured to use V_(ref) as the reference from the beginning of the restart procedure to save the time it would take for V_(soft-start) to reach V_(ref), at which time the error amplifier 34 would switch over to V_(ref) anyway.

FIG. 12 is a diagram of a portion of a boost converter power supply 130, which includes a signal-drop-reducing circuit 132 for reducing or eliminating a drop in V_(out-residual) on start or restart of the boost converter, according to an embodiment. In FIG. 12, components common to the boost converter 130 and the buck converters 10, 60, and 120 of FIGS. 1, 4, and 11 may be labeled with the same reference numbers, and, for brevity, are not described again. Furthermore, only a portion of the boost converter 130 is described below, the described portion being sufficient to explain the relevant structure, configuration, and operation of the boost converter, it being understood that the other portions of the boost converter may be similar to corresponding portions of the buck converters 10, 60, or 120. Moreover, although the boost converter 130 is described as having a single phase, it may include multiple phases.

In addition to the signal-drop-reducing circuit 132, the boost converter 130 includes a power-supply controller chip 134, a phase inductor 136, an optional feedback resistor-divider circuit 138, an output filter capacitor 140, an input node 142 configured to receive an input voltage V_(in), an output node 144 configured to provide an output voltage V_(out) to the load 12, a reference node 146 configured to receive a reference voltage (here ground), and a phase node 148.

The power-supply controller chip 134 includes soft-start circuitry (not shown in FIG. 12), an error amplifier (not shown in FIG. 12), a controller (not shown in FIG. 12), a switching-stage drive circuit 150, and a switching stage 152. The switching stage 152 and the phase inductor 136 form a respective switching phase circuit 154. And although described as being part of the controller chip 134, the switching stage 152 can be separate from the controller chip.

The drive circuit 150 converts the drive signal Drive₁ _(_) _(g) into a pair of signals Switch_(Charge) and Switch_(Discharge) for driving the switching stage 152, where Switch_(Charge)=Switch_(Discharge) , although in another embodiment, the drive circuit 150 is configured to generate Switch_(Charge) and Switch_(Discharge) so that they are both inactive (e.g., Logic 0) for a period of time to prevent a crowbar current from flowing in the switching stage 152. Furthermore, in response to signal Enable_L having a level of logic 0, the switching stage 152 is disabled and presents high impedance to the output node 144 and to the phase node 148.

The switching stage 152 has an inductor-charging NMOS transistor 156 and an inductor-discharging NMOS transistor 158. In response to Switch_(Charge)=logic 1 (e.g., 3.3 V) and Switch_(Discharge)=logic 0 (e.g., the voltage at the node 148), a linearly increasing current flows from the input node 142, through the inductor 136 and the active charging transistor 156, to the reference node 146 (here ground); because the discharging transistor 158 is inactive, negligible or no current flows to the output node 144. Conversely, in response to Switch_(Charge)=logic 0 and Switch_(Discharge)=logic 1, a linearly decreasing current flows from the input node 142, through the inductor 136 and the active discharging transistor 158, and to the output node 144; because the charging transistor 156 is inactive, negligible or no current flows through the charging transistor. And when Switch_(Charge)=Switch_(Discharge)=logic 0, both transistors 156 and 158 are inactive such that the switching stage 152 is tri-stated, i.e., the switching stage presents a high impedance to the output node 144 and the phase node 148 such that negligible or no current flows from the output node 144 into the phase node 148 or vice-versa.

The signal-drop-reducing circuit 132 takes advantage of the relationship between D, V_(in), and V_(out), which relationship, for ideal converters, is given by the following equation:

$\begin{matrix} {V_{out} = {V_{in} \cdot \frac{1}{1 - D}}} & (7) \end{matrix}$

where

$D = \frac{T_{1}}{T}$

(see FIG. 5) and the boost-converter 130 is operating in CCM.

Although during start or restart of the boost converter 130 equation (7) does not hold because the switching stage 152 presents a high impedance to the output node 144, one can still determine the equivalent output voltage, V_(out) _(_) _(eq), that the duty cycle D would generate if the switching stage 152 were active, according to the following equation:

$\begin{matrix} {V_{out\_ eq} = {V_{in} \cdot \frac{1}{1 - D}}} & (8) \end{matrix}$

That is, V_(out) _(_) _(eq) is the voltage to which D corresponds while the switching stage 152 is disabled, whereas V_(out) is the voltage to which D corresponds, and which D causes the boost converter 130 to generate on the output node 144, when the switching stage is enabled.

As described below, the signal-drop-reducing circuit 132 takes advantage of equation (8) rearranged as follows:

V _(in) =V _(out) _(_) _(eq)·(1−D)   (9)

The structure and configuration of the signal-drop-reducing circuit 132 is now described.

In addition to the first and second low-pass filters 72 and 74, the comparator 76, the latch 78, and the AND gate 80, the signal-drop-reducing circuit 132 includes an inverter 160.

The ideal inverter 160 receives the Drive₁ signal switching at duty cycle D, and due to the inverter 160 having its power rail connected to V_(out) _(_) _(residuat) it generates the inversion, or Drive₁ signal switching between V_(out) _(_) _(residual) and 0 V at duty cycle at its output.

The low-pass filter 72, which is optional, generates V_(in) _(_) _(filtered) by effectively taking the average of V_(in).

The low-pass filter 74 receives the input signal V_(out) _(_) _(residual) switching at duty cycle D and creates at its output the time average of this signal, which is V_(out) _(_) _(residual)·(1−D), as shown in equation (9), because the average of the inverse of a signal with duty cycle D is effectively equal to 1−D. As described above in conjunction with FIG. 7, depending on its frequency response, the filter 74 may generate an approximate version of V_(out) _(_) _(residual) (1−D) with a small ripple voltage.

The comparator 76 compares V_(in) _(_) _(filtered) from the first low-pass filter 72 (V_(in) _(_) _(filtered)=V_(in) if the first low-pass filter is omitted) and V_(out) _(_) _(residual)·(1−D) from the second low-pass filter 74, and generates the signal Enable having a disable value (e.g., logic 0) if V_(in) _(_) _(filtered)≦V_(out) _(_) _(residual)·(1−D) and having an enable value (e.g., logic 1) if V_(in) _(_) _(filtered)>V_(out) _(_) _(residual)·(1−D).

The latch 78 latches the signal Enable upon its transition from its disable value (e.g., logic 0) to its enable value (e.g., logic 1) to generate the signal Enable_L, which is the latched version of Enable.

And the AND gate 80, which is optional, generates the gated version Drive₁ _(_) _(g) of the drive signal Drive₁, in response to Enable_L having an enable value; when Enable_L has a disable value, the AND gate generates Drive_(1-g)=0.

Referring to FIG. 12, normal operation of the boost converter 130 in CCM is described, according to an embodiment. The operation of circuitry (e.g., error amplifier, PWM circuit) not specifically described may be similar to the operation of equivalent circuits of the buck converters 10, 60, and 120 of FIGS. 1, 4, and 11.

When Drive₁ _(_) _(g) is logic 1, the drive circuit 150 activates the transistor 156 and deactivates the transistor 158.

In response to the active transistor 156, a linearly increasing charging current flows through the inductor 136 and the transistor 156 to ground.

When Drive₁ _(_) _(g) transitions to logic 0, the drive circuit deactivates the transistor 156 and activates the transistor 158.

In response to the active transistor 158, because a current through an inductor cannot change instantaneously, the current that was flowing through the inductor 136 and the transistor 156 now flows through the inductor and the transistor 158 to the output node 144, and decreases linearly.

The boost converter 130 repeats the above cycle, and the error amplifier (not shown in FIG. 12) adjusts the duty cycle of Drive₁ as needed to maintain V_(out) at its regulated value.

Still referring to FIG. 12, operation of the boost converter 130 during a start or restart (during CCM or DCM) is described, according to an embodiment. The operation of circuitry (e.g., soft-start circuit, error amplifier, PWM circuit) not specifically described may be similar to the operation of equivalent circuits of the buck converters 10, 60, and 120 of FIGS. 1, 4, and 11.

During a start or restart of the boost converter 130, the signal-drop-reducing circuit 132 disables the switching stage 152 until V_(in) _(_) _(filtered) exceeds V_(out) _(_) _(residual)·(1−D) such that V_(out) experiences little or no voltage drop from V_(out) _(_) _(residual) upon enabling of the switching stage. In other aspects, the operation of the boost converter 130 during start or restart may be similar to the operation, during start or restart, of the buck converters 60 of FIGS. 4 and 120 of FIG. 11.

Still referring to FIG. 12, alternate embodiments of the boost converter 130 are contemplated. For example, although the signal-drop-reducing circuit 132 is described as generating V_(out) _(_) _(residual)·(1−D) from the signal Drive₁, the signal-drop-reducing circuit can generate V_(out) _(_) _(residual)(1−D) from a master PWM signal from which Drive₁ is generated. Furthermore, in a multiphase implementation, the phase inductors 136 may be magnetically uncoupled from one another, or two or more of the phase inductors may be magnetically coupled to one another. Moreover, the signal-drop-reducing circuit 132 can include the circuit 100 of FIG. 10 or an equivalent circuit. In addition, the signal-drop-reducing circuit 132 can include a low-pass filter to filter V_(out) _(_) _(residual) and to provide the filtered V_(out) _(_) _(residual) to the power node of the inverter 160. In addition, the boost converter 130 can include circuitry similar to the V_(in) and V_(out-residual) level shifters 90 and 92 of FIG. 8. Furthermore, instead of including two NMOS transistors 156 and 158 as switches, each switching stage 152 may include a PMOS transistor for the charging switch, for the discharging switch, or both the charging and discharging switches.

FIG. 13 is a diagram of a portion of a buck-boost converter power supply 170, which includes a signal-drop-reducing circuit 172 for reducing or eliminating a drop in V_(out-residual) on start or restart of the buck-boost converter, according to an embodiment. In FIG. 13, components common to the buck-boost converter 170 and the buck converters 10, 60, and 120 of FIGS. 1, 4, and 11 and the boost converter 130 of FIG. 12 may be labeled with the same reference numbers, and, for brevity, are not described in detail again. Furthermore, only a portion of the buck-boost converter 170 is described below, the described portion being sufficient to explain the relevant structure, configuration, and operation of the buck-boost converter, it being understood that the other portions of the buck-boost converter may be similar to corresponding portions of the buck converters 10, 60, or 120 or the boost converter 130. Moreover, although the buck-boost converter 170 is described as having a single phase, it may include multiple phases.

In addition to the signal-drop-reducing circuit 172, the buck-boost converter 170 includes a power-supply controller chip 174, a phase inductor 176, an optional feedback resistor-divider circuit 178, an output filter capacitor 180, an input node 182 configured to receive an input voltage V_(in), an output node 184 configured to provide an output voltage V_(out), a reference node 186 configured to receive a reference voltage (here ground), and a phase node 188.

The power-supply controller chip 174 includes soft-start circuitry (not shown in FIG. 13), an error amplifier (not shown in FIG. 13), a controller (not shown in FIG. 13), a switching-stage drive circuit 190, and a switching stage 192. The switching stage 192 and the phase inductor 176 form a respective switching phase circuit 194. And although described as being part of the controller chip 174, the switching stage 192 can be separate from the controller chip.

The drive circuit 190 is configured to convert the drive signal Drive₁ _(_) _(g) into a pair of signals Switch_(Charge) and Switch_(Discharge) for driving the switching stage 192, where Switch_(Charge)=Switch_(Discharge) , although in another embodiment, the drive circuit 190 is configured to generate Switch_(Charge) and Switch_(Discharge) so that they are both inactive (e.g., Logic 0) for a period of time to prevent a current from flowing directly between V_(in) and V_(out) as is conventional. Furthermore, in response to signal Enable_L having a level of logic 0, the switching stage 192 is disabled and presents high impedance to the output node 184 and to the phase node 188.

The switching stage 192 has an inductor-charging NMOS transistor 196 and an inductor-discharging NMOS transistor 198. In response to Switch_(Charge)=logic 1 (e.g., the voltage at the node 188+3.3 V) and Switch_(Discharge)=logic 0 (e.g., the voltage V_(out) at the node 184), a linearly increasing current flows from the input node 182, through the active charging transistor 196 and the inductor 176, to the reference node 186 (here ground); because the discharging transistor 198 is inactive, negligible or no current flows to the output node 184. Conversely, in response to Switch_(Charge)=logic 0 and Switch_(Discharge)=logic 1, a linearly decreasing current flows from the output node 184, through the active discharging transistor 198 and the inductor 176, to ground; because the charging transistor 196 is inactive, negligible or no current flows through the charging transistor. And when Switch_(Charge)=Switch_(Discharge)=logic 0, both transistors 196 and 198 are inactive such that the switching stage 192 is tri-stated, i.e., the switching stage presents a high impedance to the output node 184 and the phase node 188 such that negligible or no current flows from the output node 184 into the phase node 188 and vice-versa.

The signal-drop-reducing circuit 172 takes advantage of the relationship between D, V_(in), and V_(out), which relationship, for an ideal buck-boost converter, is given by the following equation:

$\begin{matrix} {V_{out} = {V_{in} \cdot \frac{- D}{1 - D}}} & (10) \end{matrix}$

where

$D = \frac{T_{1}}{T}$

(see FIG. 5) and the buck-boost converter 170 is operating in CCM.

Although during start or restart of the buck-boost converter 170 equation (10) does not hold because the switching stage 192 presents a high impedance to the output node 184, one can still determine the equivalent output voltage, V_(out) _(_) _(eq), that the duty cycle D would generate if the switching stage 192 were active, according to the following equation for an ideal buck-boost converter:

$\begin{matrix} {V_{out\_ eq} = {V_{in} \cdot \frac{- D}{1 - D}}} & (11) \end{matrix}$

That is, V_(out) _(_) _(eq) is the voltage to which D corresponds when the switching stage 192 is disabled, whereas V_(out) is the voltage to which D corresponds, and which D causes the buck-boost converter 170 to generate on the output node 184, while the switching stage is enabled.

As described below, signal-drop-reducing circuit 172 takes advantage of equation (11) rearranged as follows:

V _(in) ·D=−V _(out) _(_) _(eq)·(1−D)   (12)

The structure and configuration of the signal-drop-reducing circuit 172 is now described.

In addition to the first and second low-pass filters 72 and 74, the comparator 76, the latch 78, the AND gate 80, and the inverter 160, the signal-drop-reducing circuit 172 includes an amplifier 200 with a gain of −1, and a logic buffer 204, which receives V_(in) on a supply node.

The inverter 160 generates the inversion of the Drive₁ signal switching between −V_(out) _(_) _(residual) and 0 V at its output.

The low-pass filter 74 provides at its output the time average of its input, which would be −V_(out) _(_) _(residual)·(1−D) of equation (12) because the average of the inverse of a signal with duty cycle D is equal to 1−D. As described above in conjunction with FIG. 7, depending on its frequency response, the filter 74 may generate an approximate version of −V_(out) _(_) _(residual)·(1−D) with a small ripple voltage.

The buffer 204 generates the Drive₁ signal at duty cycle D switching between 0 V and V_(in).

The low-pass filter 72 generates V_(in)·D per equation (12) because the average of a signal that switches between 0 V and V_(in) at a duty cycle D is equal to V_(in)·D.

The comparator 76 compares V_(in)·D from the first low-pass filter 72 and −V_(out) _(_) _(residual)·(1−D) from the second low-pass filter 74, and generates the signal Enable having a disable value (e.g., logic 0) if V_(in)·D≦−V_(out) _(_) _(residual)·(1−D) and having an enable value (e.g., logic 1) if V_(in)·D>−V_(out) _(_) _(residual)·(1−D).

The latch 78 latches the signal Enable upon its transition from its disable value (e.g., logic 0) to its enable value (e.g., logic 1) to generate the signal Enable_L, which is the latched version of Enable.

And the AND gate 80, which is optional, generates the gated version Drive₁ _(_) _(g) of the drive signal Drive₁, in response to Enable_L having an enable value; when Enable_L has a disable value, the AND gate generates Drive_(1-g)=0.

Referring to FIG. 13, normal operation of the buck-boost converter 170 in CCM is described, according to an embodiment. The operation of circuitry (e.g., error amplifier, PWM circuit) not specifically described may be similar to the operation of equivalent circuits of the buck converters 10, 60, or 120 of FIGS. 1, 4, and 11 or of the boost converter 130 of FIG. 12.

When Drive₁ _(_) _(g) is logic 1, the drive circuit 190 activates the transistor 196 and deactivates the transistor 198.

In response to the active transistor 196, a linearly increasing charging current flows through the transistor 196 and the inductor 176 to ground.

When Drive₁ _(_) _(g) transitions to logic 0, the drive circuit deactivates the transistor 196 and activates the transistor 198.

In response to the active transistor 198, because a current through an inductor cannot change instantaneously, the current that was flowing through the inductor 176 and the transistor 196 now flows from the output node 184, through the transistor 198 and the inductor 176, to ground, and decreases linearly (the polarity of V_(out) is opposite the polarity of V_(in)).

The buck-boost converter 170 repeats the above cycle, and the error amplifier (not shown in FIG. 13) adjusts the duty cycle of Drive₁ as needed to maintain V_(out) at its regulated value.

Still referring to FIG. 13, operation of the buck-boost converter 170 during a start or restart (during CCM or DCM) is described, according to an embodiment. The operation of circuitry (e.g., soft-start circuit, error amplifier, PWM circuit) not specifically described may be similar to the operation of equivalent circuits of the buck converters 10, 60, or 120 of FIGS. 1, 4, and 11 or of the boost converter 130 of FIG. 12.

During a start or restart of the buck-boost converter 170, the signal-drop-reducing circuit 172 disables the switching stage 192 until V_(in)·D exceeds −V_(out) _(_) _(residual)·(1−D) such that V_(out) experiences little or no voltage drop from V_(out) _(_) _(residual) upon enabling of the switching stage. In other aspects, the operation of the buck-boost converter 170 during start or restart may be similar to the operation of the buck converters 60 of FIG. 4 or 120 of FIG. 11 or of the boost converter 130 of FIG. 12.

Still referring to FIG. 13, alternate embodiments of the buck-boost converter 170 are contemplated. For example, although the signal-drop-reducing circuit 172 is described as generating −V_(out) _(_) _(residual)·(1−D) and V_(in)·D from the signal Drive₁, the signal-drop-reducing circuit can generate −V_(out) _(_) _(residual)·(1−D) and V_(in)·D from a master PWM signal from which Drive₁ is generated. Furthermore, in a multiphase implementation, the phase inductors 176 may be magnetically uncoupled from one another, or two or more of the phase inductors may be magnetically coupled to one another. Moreover, the signal-drop-reducing circuit 172 can include the circuit 100 of FIG. 10 or an equivalent circuit. In addition, the signal-drop-reducing circuit can include a low-pass filter to filter V_(out) _(_) _(residual) and to provide the filtered V_(out) _(_) _(residual) to the amplifier 200. Furthermore, the buck-boost converter 170 can include circuitry similar to the V_(in) and V_(out-residual) level shifters 90 and 92 of FIG. 8. Moreover, instead of including two NMOS transistors 196 and 198 as switches, each switching stage 192 may include a PMOS transistor for the charging switch, for the discharging switch, or both the charging and discharging switches.

FIG. 14 is a diagram of a portion of a single-ended primary-inductor converter (SEPIC) 210, which includes a signal-drop-reducing circuit 212 for reducing or eliminating a drop in V_(out-residual) on start or restart of the SEPIC, according to an embodiment. In FIG. 14, components common to the SEPIC 210 and the buck converters 10, 60, and 120 of FIGS. 1, 4, and 11, the boost converter 130 of FIG. 12, and the buck-boost converter 170 of FIG. 13 may be labeled with the same reference numbers, and, for brevity, are not described in detail again. Furthermore, only a portion of the SEPIC 210 is described below, the described portion being sufficient to explain the relevant structure and operation of the SEPIC, it being understood that the other portions of the SEPIC may be similar to corresponding portions of the buck converters 10, 60, or 120, the boost converter 130, or the buck-boost converter 170. Moreover, although the SEPIC 210 is described as having a single phase, it may include multiple phases.

In addition to the signal-drop-reducing circuit 212, the SEPIC 210 includes a power-supply controller chip 214, phase inductors 216 and 217, an optional feedback resistor-divider circuit 218, a phase capacitor 219, an output filter capacitor 220, an input node 222 configured to receive an input voltage V_(in), an output node 224 configured to provide an output voltage V_(out) to the load 12, a reference node 226 configured to receive a reference voltage (here ground), and phase nodes 228 and 229.

The power-supply controller chip 214 includes soft-start circuitry (not shown in FIG. 14), an error amplifier (not shown in FIG. 14), a controller (not shown in FIG. 14), a switching-stage drive circuit 230, and a switching stage 232. The switching stage 232, the phase inductors 216 and 217, and the phase capacitor 219 form a respective switching phase circuit 234. And although described as being part of the controller chip 214, the switching stage 232 can be separate from the controller chip.

The drive circuit 230 is configured to convert the drive signal Drive₁ _(_) _(g) into a pair of signals Switch_(Charge) and Switch_(Discharge) for driving the switching stage 232, where Switch_(Charge)=Switch_(Discharge) , although in another embodiment, the drive circuit 230 is configured to generate Switch_(Charge) and Switch_(Discharge) so that they are both inactive (e.g., Logic 0) for a period of time to prevent a current from flowing from the output node 224 to either V_(in) or ground. Furthermore, in response to signal Enable_L having a level of logic 0, the switching stage 232 is disabled and presents high impedances to the phase nodes 228 and 229 and to the output node 224.

The switching stage 232 has an inductor-charging NMOS transistor 236 and an inductor-discharging NMOS transistor 238. In response to Switch_(Charge)=logic 1 (e.g., 3.3 V) and Switch_(Discharge)=logic 0 (e.g., the voltage at the node 229) during a first cycle, a linearly increasing current flows from the input node 222, through the phase inductor 216 and the active charging transistor 236, to the reference node 226 (here ground); because the discharging transistor 238 is inactive, negligible or no current flows to the output node 224. Conversely, in response to Switch_(Charge)=logic 0 and Switch_(Discharge)=logic 1 during the first cycle, a linearly decreasing current flows from the input node 222, through the inductor 216, capacitor 219 (this current generates a voltage across the capacitor), and the active discharging transistor 238, to the output node 224; because the charging transistor 236 is inactive, negligible or no current flows through the charging transistor. In response to Switch_(Charge)=logic 1 and Switch_(Discharge)=logic 0 during a second and subsequent cycles, because the capacitor 219 now has a voltage across it, a first linearly increasing current flows from the input node 222, through the phase inductor 216 and the active charging transistor 236, to the reference node 226 (here ground), and a second linearly increasing current flows from the reference node, through the second phase inductor 217, capacitor 219, and active charging transistor to the reference node; because the discharging transistor 238 is inactive, negligible or no current flows to the output node 224. Conversely, in response to Switch_(Charge)=logic 0 and Switch_(Discharge)=logic 1 during the second and subsequent cycles, a first linearly decreasing current flows from the input node 222, through the inductor 216, capacitor 219, and the active discharging transistor 238, to the output node 224, and a second linearly decreasing current flows from the reference node 226, through the second phase inductor 217 and active discharging transistor, to the output node; because the charging transistor 236 is inactive, negligible or no current flows through the charging transistor. And when Switch_(Charge)=Switch_(Discharge)=logic 0, both transistors 236 and 238 are inactive such that the switching stage 232 is tri-stated, i.e., the switching stage presents a high impedance to the output node 224 and the phase nodes 228 and 229 such that negligible or no current flows from the output node 224 into the phase nodes and vice-versa.

The signal-drop-reducing circuit 212 takes advantage of the relationship between D, V_(in), and V_(out), which relationship, for an ideal SEPIC converter, is given by the following equation:

$\begin{matrix} {V_{out} = {V_{in} \cdot \frac{D}{1 - D}}} & (13) \end{matrix}$

where

$D = \frac{T_{1}}{T}$

(see FIG. 5) and the SEPIC 210 is operating in CCM.

Although during start or restart of the SEPIC 210 equation (13) does not hold because the switching stage 232 presents a high impedance to the output node 224, one can still determine the equivalent output voltage, V_(out) _(_) _(eq), that the duty cycle D would generate if the switching stage 232 were active, according to the following equation for an ideal SEPIC converter:

$\begin{matrix} {V_{out\_ eq} = {V_{in} \cdot \frac{D}{1 - D}}} & (14) \end{matrix}$

That is, V_(out) _(_) _(eq) is the voltage to which D corresponds when the switching stage 232 is disabled, whereas V_(out) is the voltage to which D corresponds, and which D causes the SEPIC 210 to generate on the output node 224, when the switching stage is enabled.

As described below, signal-drop-reducing circuit 212 takes advantage of equation (14) rearranged as follows:

V _(in) ·D=V _(out) _(_) _(eq)·(1−D)   (15)

The structure and configuration of the signal-drop-reducing circuit 212 is now described.

The signal-drop-reducing circuit 212 is similar to the signal-drop-reducing circuit 172 of FIG. 13, except that the circuit 212 does not include the amplifier 200. That is, the signal-drop-reducing circuit 212 includes the first and second low-pass filters 72 and 74, the comparator 76, the latch 78, the AND gate 80, the inverter 160, and the logic buffer 204.

The inverter 160 generates the signal Drive₁ switching between 0 V and V_(out) _(_) _(residual)at a duty cycle D.

The low-pass filter 74 converts the output from the inverter 160 into V_(out) _(_) _(residual)·(1−D) from equation (15) because the average of a signal switching between 0 V and V_(out) _(_) _(residual) at a duty cycle D is equal to V_(out) _(_) _(residual) (1−D). As described above in conjunction with FIG. 7, depending on its frequency response, the filter 74 may generate an approximate version of V_(out) _(_) _(residual)·(1−D) with a small ripple voltage.

The buffer 204 generates the signal Drive₁ switching between 0 V and V_(in) at the duty cycle D.

The low-pass filter 72 generates V_(in)·D from equation (15) because the average of a 0−V_(in) signal with duty cycle D is effectively equal to V_(in)·D.

The comparator 76 compares V_(in)·D from the first low-pass filter 72 and V_(out) _(_) _(residual)·(1−D) from the second low-pass filter 74, and generates the signal Enable having a disable value (e.g., logic 0) if V_(in)·D≦V_(out) _(_) _(residual)·(1−D) and having an enable value (e.g., logic 1) if V_(in)·D>V_(out) _(_) _(residual)·(1−D).

The latch 78 latches the signal Enable upon its transition from its disable value (e.g., logic 0) to its enable value (e.g., logic 1) to generate the signal Enable_L, which is the latched version of Enable.

And the AND gate 80, which is optional, generates the gated version Drive₁ _(_) _(g) of the drive signal Drive₁, in response to Enable_L having an enable value; when Enable_L has a disable value, the AND gate generates Drive_(1-g)=0.

Referring to FIG. 14, normal operation of the SEPIC 210 in CCM is described, according to an embodiment. The operation of circuitry (e.g., error amplifier, PWM circuit) not specifically described may be similar to the operation of equivalent circuits of the buck converters 10, 60, or 120 of FIGS. 1, 4, and 11, of the boost converter 130 of FIG. 12, or of the buck-boost converter 170 of FIG. 13. Furthermore, the description is limited to the second and subsequent PWM cycles after starting of the SEPIC 210.

When Drive₁ _(_) _(g) is logic 1, the drive circuit 230 activates the transistor 236 and deactivates the transistor 238.

In response to the active transistor 236, a first linearly increasing charging current flows through the transistor 236 and the first phase inductor 216 to ground, and a second linearly increasing charging current flows from ground, through the second phase inductor 217 and phase capacitor 219, through the transistor 236, to ground.

When Drive₁ _(_) _(g) transitions to logic 0, the drive circuit 230 deactivates the transistor 236 and activates the transistor 238.

In response to the active transistor 238, because a current through an inductor cannot change instantaneously, the current that was flowing through the first phase inductor 216 and the transistor 236 now flows through the first phase inductor, the phase capacitor 219, and the transistor 238 to the output node 224, and the current that was flowing through the second phase inductor 217 and the transistor 236 now flows through the second phase inductor and the transistor 238 to the output node; both of these currents decrease linearly. And it is noted that during the first PWM cycle, there is no current flowing through the second phase inductor 217.

The SEPIC 210 repeats the above cycle, and the error amplifier (not shown in FIG. 14) adjusts the duty cycle of Drive₁ as needed to maintain V_(out) at its regulated value.

Still referring to FIG. 14, operation of the SEPIC 210 during a start or restart (during CCM or DCM) is described, according to an embodiment. The operation of circuitry (e.g., soft-start circuit, error amplifier, PWM circuit) not specifically described may be similar to the operation of equivalent circuits of the buck converters 10, 60, or 120 of FIGS. 1, 4, and 11, of the boost converter 130 of FIG. 12, or of the buck-boost converter 170 of FIG. 13.

During a start or restart of the SEPIC 210, the signal-drop-reducing circuit 212 disables the switching stage 232 until V_(in)·D exceeds V_(out) _(_) _(residual)·(1−D) such that V_(out) experiences little or no voltage drop from V_(out) _(_) _(residual) upon enabling of the switching stage. In other aspects, the operation of the SEPIC 210 during start or restart may be similar to the operation of the buck converters 60 of FIG. 4 or 120 of FIG. 11, of the boost converter 130 of FIG. 12, or of the buck-boost converter 170 of FIG. 13.

Still referring to FIG. 14, alternate embodiments of the SEPIC 210 are contemplated. For example, although the signal-drop-reducing circuit 212 is described as generating V_(out) _(_) _(residual)·(1−D) and V_(in)·D from the signal Drive₁, the signal-drop-reducing circuit can generate V_(out) _(_) _(residual)·(1−D) and V_(in)·D from a master PWM signal from which Drive₁ is generated. Furthermore, in a multiphase implementation, the phase inductors 216 and 217 may be magnetically uncoupled from one another, or two or more of the phase inductors may be magnetically coupled to one another. Moreover, the signal-drop-reducing circuit 212 can include the circuit 100 of FIG. 10 or an equivalent circuit. In addition, the signal-drop-reducing circuit 212 can include a low-pass filter to filter V_(out) _(_) _(residual) and to provide the filtered V_(out-residual) to the supply node of the inverter 160. Furthermore, the SEPIC 210 can include circuitry similar to the V_(in) and V_(out-residual) level shifters 90 and 92 of FIG. 8. Moreover, instead of including two NMOS transistors 236 and 238 as switches, each switching stage 232 may include a PMOS transistor for the charging switch, for the discharging switch, or both the charging and discharging switches.

FIG. 15 is a diagram of a portion of a Ćuk (pronounced “chuk”) converter 250, which includes a signal-drop-reducing circuit 252 for reducing or eliminating a drop in V_(out-residual) on start or restart of the Ćuk converter, according to an embodiment. In FIG. 15, components common to the Ćuk converter 250 and the buck converters 10, 60, and 120 of FIGS. 1, 4, and 11, the boost converter 130 of FIG. 12, the buck-boost converter 170 of FIG. 13, and the SEPIC 210 of FIG. 14 may be labeled with the same reference numbers, and, for brevity, are not described again in detail. Furthermore, only a portion of the Ćuk converter 250 is described below, the described portion being sufficient to explain the relevant structure and operation of the Ćuk converter, it being understood that the other portions of the Ćuk converter may be similar to corresponding portions of the buck converters 10, 60, or 120, the boost converter 130, the buck-boost converter 170, or the SEPIC 210. Moreover, although the Ćuk converter 250 is described as having a single phase, it may include multiple phases.

In addition to the signal-drop-reducing circuit 252, the Ćuk converter 250 includes a power-supply controller chip 254, phase inductors 256 and 257, an optional feedback resistor-divider circuit 258, a phase capacitor 259, an output filter capacitor 260, an input node 262 configured to receive an input voltage V_(in), an output node 264 configured to provide an output voltage V_(out), a reference node 266 configured to receive a reference voltage (here ground), and phase nodes 268 and 269.

The power-supply controller chip 254 includes soft-start circuitry (not shown in FIG. 15), an error amplifier (not shown in FIG. 15), a controller (not shown in FIG. 15), a switching-stage drive circuit 270, and a switching stage 272. The switching stage 272, the phase inductors 256 and 257, and the phase capacitor 259 form a respective switching phase circuit 274. And although described as being part of the controller chip 254, the switching stage 272 can be separate from the controller chip.

The drive circuit 270 is configured to convert the drive signal Drive₁ _(_) _(g) into a pair of signals Switch_(Charge) and Switch_(Discharge) for driving the switching stage 272, where Switch_(Charge)=Switch_(Discharge) , although in another embodiment, the drive circuit 270 is configured to generate Switch_(Charge) and Switch_(Discharge) so that they are both inactive (e.g., Logic 0) for a period of time to prevent a current from flowing from the output node 264 to V_(in) or ground. Furthermore, in response to signal Enable_L having a level of logic 0, the switching stage 272 is disabled and presents high impedances to the phase nodes 268 and 269 and to the output node 264.

The switching stage 272 has an inductor-charging NMOS transistor 276 and an inductor-discharging NMOS transistor 278. In response to Switch_(Charge)=logic 1 (e.g., 3.3 V) and Switch_(Discharge)=logic 0 (e.g., the voltage at the node 269) during a first cycle, a linearly increasing current flows from the input node 262, through the phase inductor 256 and the active charging transistor 276, to the reference node 266 (here ground); because the discharging transistor 278 is inactive, negligible or no current flows through the discharging transistor. Conversely, in response to Switch_(Charge)=logic 0 and Switch_(Discharge)=logic 1 during the first cycle, a linearly decreasing current flows from the input node 262, through the inductor 256, capacitor 259 (this current generates a voltage across the capacitor), and the active discharging transistor 278, to ground; because the charging transistor 276 is inactive, negligible or no current flows through the charging transistor. In response to Switch_(Charge)=logic 1 and Switch_(Discharge)=logic 0 during a second and subsequent cycles, because the capacitor 259 now has a voltage across it, a first linearly increasing current flows from the input node 262, through the phase inductor 256 and the active charging transistor 276, to the reference node 226 (here ground), and a second linearly increasing current flows from the output node 264, through the second phase inductor 257, capacitor 259, and active charging transistor to the reference node; because the discharging transistor 278 is inactive, negligible or no current flows through the discharging transistor. Conversely, in response to Switch_(Charge)=logic 0 and Switch_(Discharge)=logic 1 during the second and subsequent cycles, a first linearly decreasing current flows from the input node 262, through the inductor 256, capacitor 259, and the active discharging transistor 278, to the reference node 266, and a second linearly decreasing current flows from the output node 264, through the second phase inductor 257 and active discharging transistor, to the reference node; because the charging transistor 276 is inactive, negligible or no current flows through the charging transistor. And when Switch_(Charge)=Switch_(Discharge)=logic 0, both transistors 276 and 278 are inactive such that the switching stage 272 is tri-stated, i.e., the switching stage presents high impedances to the output node 264 and the phase nodes 268 and 269 such that negligible or no current flows from the output node 264 into the phase nodes or vice-versa (the phase capacitor 259 also acts to prevent a DC current from flowing to or from the output node while the switching stage is disabled).

The signal-drop-reducing circuit 252 takes advantage of the relationship between D, V_(in), and V_(out), which relationship, for an ideal Ćuk converter, is given by the following equation:

$\begin{matrix} {V_{out} = {V_{in} \cdot \frac{- D}{1 - D}}} & (16) \end{matrix}$

where

$D = \frac{T_{1}}{T}$

(see FIG. 5) and the Ćuk converter 250 is operating in CCM.

Although during start or restart of the Ćuk converter 250 equation (16) does not hold because the switching stage 272 presents a high impedance to the output node 264, one can still determine the equivalent output voltage, V_(out) _(_) _(eq), that the duty cycle D would generate if the switching stage 272 were active, according to the following equation for an ideal Ćuk converter:

$\begin{matrix} {V_{out\_ eq} = {V_{in} \cdot \frac{- D}{1 - D}}} & (17) \end{matrix}$

That is, V_(out) _(_) _(eq) is the voltage to which D corresponds when the switching stage 272 is disabled, whereas V_(out) is the voltage to which D corresponds, and which D causes the Ćuk converter 250 to generate on the output node 264, when the switching stage is enabled.

As described below, signal-drop-reducing circuit 252 takes advantage of equation (17) rearranged as follows:

V _(in) ·D=−V _(out) _(_) _(eq)·(1−D)   (18)

The structure and configuration of the signal-drop-reducing circuit 252 is now described.

The signal-drop-reducing circuit 252 is similar to the signal-drop-reducing circuit 172 of FIG. 13. That is, the signal-drop-reducing circuit 252 includes the first and second low-pass filters 72 and 74, the comparator 76, the latch 78, the AND gate 80, the inverter 160, the amplifier 200, and the buffer 204.

The inverter 160 generates the inversion of the signal Drive₁, which transitions between 0 V and −V_(out) _(_) _(residual) at a duty cycle D.

The low-pass filter 74 converts the signal output from the inverter 160 into −V_(out) _(_) _(residual)·(1−D) of equation (18) because the average of the inverse of a 0 −V_(out) _(_) _(residual) signal with duty cycle D is effectively equal to −V_(out) _(_) _(residual)·(1−D). As described above in conjunction with FIG. 7, depending on its frequency response, the filter 74 may generate an approximate version of −V_(out) _(_) _(residual)·(1−D) with a small ripple voltage.

The buffer 204 generates the signalDrive₁ switching between 0 V and V_(in) at the duty cycle D.

The low-pass filter 72 generates V_(in)·D of equation (18) because the average of a 0−V_(in) signal with duty cycle D is effectively equal to V_(in)·D.

The comparator 76 compares V_(in)·D from the first low-pass filter 72 and −V_(out) _(_) _(residual)·(1−D) from the second low-pass filter 74, and generates the signal Enable having a disable value (e.g., logic 0) if V_(in)·D≦−V_(out) _(_) _(residual)·(1−D) and having an enable value (e.g., logic 1) if V_(in)·D>−V_(out) _(_) _(residual)·(1−D) (the Ćuk converter 250 generates V_(out) havng the opposite polarity relative to V_(in)).

The latch 78 latches the signal Enable upon its transition from its disable value (e.g., logic 0) to its enable value (e.g., logic 1) to generate the signal Enable_L, which is the latched version of Enable.

And the AND gate 80, which is optional, generates the gated version Drive₁ _(_) _(g) of the drive signal Drive₁, in response to Enable_L having an enable value; when Enable_L has a disable value, the AND gate generates Drive_(1-g)=0.

Referring to FIG. 15, normal operation of the Ćuk converter 250 in CCM is described, according to an embodiment. The operation of circuitry (e.g., error amplifier, PWM circuit) not specifically described may be similar to the operation of equivalent circuits of the buck converters 10, 60, or 120 of FIGS. 1, 4, and 11, of the boost converter 130 of FIG. 12, of the buck-boost converter 170 of FIG. 13, or of the SEPIC 210 of FIG. 14. Furthermore, the description is limited to the second and subsequent PWM cycles after starting of the Ćuk converter 250.

When Drive₁ _(_) _(g) is logic 1, the drive circuit 270 activates the transistor 276 and deactivates the transistor 278.

In response to the active transistor 276, a first linearly increasing charging current flows through the transistor 276 and the first phase inductor 256 to ground, and a second linearly increasing charging current flows from the output node 264, through the second phase inductor 257, phase capacitor 259, and transistor 236, to ground.

When Drive₁ _(_) _(g) transitions to logic 0, the drive circuit 270 deactivates the transistor 276 and activates the transistor 278.

In response to the active transistor 278, because a current through an inductor cannot change instantaneously, the current that was flowing through the first phase inductor 256 and the transistor 276 now flows through the first phase inductor, the phase capacitor 259, and the transistor 278 to ground, and the current that was flowing through the second phase inductor 257, phase capacitor 259, and transistor 276 now flows through the second phase inductor and the transistor 278 to ground; both of these currents decrease linearly. And it is noted that during the first PWM cycle, there is no current flowing through the second phase inductor 257.

The Ćuk converter 250 repeats the above cycle, and the error amplifier (not shown in FIG. 15) adjusts the duty cycle of Drive₁ as needed to maintain V_(out) at its regulated value.

Still referring to FIG. 15, operation of the Ćuk converter 250 during a start or restart (during CCM or DCM) is described, according to an embodiment. The operation of circuitry (e.g., soft-start circuit, error amplifier, PWM circuit) not specifically described may be similar to the operation of equivalent circuits of the buck converters 10, 60, or 120 of FIGS. 1, 4, and 11, of the boost converter 130 of FIG. 12, of the buck-boost converter 170 of FIG. 13, or of the SEPIC 210 of FIG. 14.

During a start or restart of the Ćuk converter 250, the signal-drop-reducing circuit 252 disables the switching stage 272 until V_(in)·D exceeds −V_(out) _(_) _(residual)·(1−D) such that V_(out) experiences little or no voltage drop from V_(out) _(_) _(residual) upon enabling of the switching stage. In other aspects, the operation of the Ćuk converter 250 during start or restart may be similar to the operation of the buck converters 60 of FIG. 4 or 120 of FIG. 11, of the boost converter 130 of FIG. 12, of the buck-boost converter 170 of FIG. 13, or of the SEPIC 210 of FIG. 14.

Still referring to FIG. 15, alternate embodiments of the Ćuk converter 250 are contemplated. For example, although the signal-drop-reducing circuit 252 is described as generating −V_(out) _(_) _(residual)·(1−D) and V_(in)·D from the signal Drive₁, the signal-drop-reducing circuit can generate −V_(out) _(_) _(residual)·(1−D) and V_(in)·D from a master PWM signal from which Drive₁ is generated. Furthermore, in a multiphase implementation, the phase inductors 256 and 257 may be magnetically uncoupled from one another, or two or more of the phase inductors may be magnetically coupled to one another. Moreover, the signal-drop-reducing circuit 252 can include the circuit 100 of FIG. 10 or an equivalent circuit. In addition, the signal-drop-reducing circuit 252 can include a low-pass filter to filter V_(out) _(_) _(residual) and to provide the filtered V_(out-residual) to the amplifier 200. Furthermore, the Ćuk converter 250 can include circuitry similar to the V_(in) and V_(out-residual) level shifters 90 and 92 of FIG. 8. Moreover, instead of including two NMOS transistors 276 and 278 as switches, each switching stage 272 may include a PMOS transistor for the charging switch, for the discharging switch, or both the charging and discharging switches.

FIG. 16 is a block diagram of an embodiment of a computer system 280, which incorporates, as a power supply 282, one or more of the buck converters of FIGS. 4 and 11, the boost converter 130 of FIG. 12, the buck-boost converter 170 of FIG. 13, the SEPIC 210 of FIG. 14, or the Ćuk converter 250 of FIG. 16, according to an embodiment. Although the system 280 is described as a computer system, it may be any system for which an embodiment of the power supply 282 is suited.

The system 280 includes computing circuitry 284, which, in addition to the power supply 282, includes a processor 286 (e.g., a microprocessor or microcontroller) powered by the supply (i.e., the processor is a load of the supply), at least one input device 288, at least one output device 290, and at least one data-storage device 292.

In addition to processing data, the processor 286 may program or otherwise control the power supply 282. For example, one or more of the functions of the power-supply controller of the power supply may be performed by the processor 286.

The input device (e.g., keyboard, mouse) 288 allows the providing of data, programming, and commands to the computing circuitry 284.

The output device (e.g., display, printer, speaker) 290 allows the computing circuitry 284 to provide data in a form perceivable by a human operator.

And the data-storage device (e.g., flash drive, hard disk drive, RAM, optical drive) 292 allows for the storage of, e.g., programs and data.

Still referring to FIG. 16, although described as powering the processor 286, the power supply 282 may provide power to one or more other components of the system 280 instead of, or in addition to, the processor. Furthermore, one or more components (e.g., the power-supply controller) of the power supply 282 may be disposed on a same integrated circuit die as other components (e.g., the processor 286) of the system 280.

EXAMPLE EMBODIMENTS

Example 1 includes a system, comprising: a power supply, including an input node configured to receive an input signal, an output node, a power-supply phase circuit configured to generate an output signal on the output node in response to the input signal, the output signal having a magnitude, a control circuit configured to drive the first power-supply phase circuit with a duty cycle, and a signal-drop-reducing circuit configured to disable the first power-supply phase circuit in response to the duty cycle corresponding to a signal magnitude that is lower than the magnitude of the output signal; and a load coupled to the output node of the power supply.

Example 2 includes the system of Examples 1 wherein the power supply includes a buck converter.

Example 3 includes the system of any of Examples 1-2 wherein the power supply includes a boost converter.

Example 4 includes the system of any of Examples 1-3 wherein the power supply includes a buck-boost converter.

Example 5 includes the system of any of Examples 1-4 wherein the power supply includes a SEPIC converter.

Example 6 includes the system of any of Examples 1-5 wherein the power supply includes a ĆUK converter.

Example 7 includes the system of any of Examples 1-6 wherein the load includes a microprocessor or a microcontroller.

Example 8 includes a method, comprising: generating a pulse-width-modulated signal having a duty cycle; and isolating a power-supply output node in response to the duty cycle corresponding to a signal magnitude that is less than a magnitude of an output signal on the power-supply output node.

Example 9 includes the method of Example 8 wherein generating the pulse-width-modulated signal includes generating the pulse-width-modulated signal in response to a difference between a reference signal and a feedback signal related to the output signal.

Example 10 includes the method of any of Examples 8-9 wherein generating the pulse-width-modulated signal includes generating the pulse-width-modulated signal in response to a difference between a soft-start signal and a feedback signal related to the output signal.

Example 11 includes the method of any of Examples 8-10 wherein isolating the power-supply output node includes isolating the power-supply output node in response to the duty cycle corresponding to a signal magnitude that is equal to a magnitude of an output signal on the power-supply output node.

Example 12 includes the method of any of Examples 8-11, further comprising driving the power-supply output node in response to the pulse-width-modulated signal and in response to the duty cycle corresponding to a signal magnitude that is greater than the magnitude of the output signal.

Example 13 includes the method of any of Examples 8-12 wherein: isolating the power-supply output node includes tri-stating a power-supply phase circuit; and wherein driving the power-supply output node includes activating the power-supply phase circuit.

Example 14 includes the method of any of Examples 8-13, further comprising: comparing a representative signal related to the output signal to the signal magnitude; and wherein isolating the power-supply output node includes isolating the power-supply output node in response to a magnitude of the representative signal being greater than the signal magnitude.

Example 15 includes the method of any of Examples 8-14, further comprising: generating, in response to the pulse-width-modulated signal, a filtered version of the pulse-width-modulated signal, the signal magnitude equaling an amplitude of the filtered version of the pulse-width-modulated signal; comparing a representative signal related to the output signal to the filtered version of the pulse-width-modulated signal; wherein isolating the power-supply output node includes isolating the power-supply output node in response to an amplitude of the representative signal being greater than the amplitude of the filtered version of the pulse-width-modulated signal; and driving the power-supply output node in response to the amplitude of the representative signal being less than the amplitude of the filtered version of the pulse-width-modulated signal.

Example 16 includes the method of any of Examples 8-15 wherein the representative signal equals the output signal.

Example 17 includes the method of any of Examples 8-16, further comprising: generating, in response to the pulse-width-modulated signal, a filtered version of the pulse-width-modulated signal, the signal magnitude equaling an amplitude of the filtered version of the pulse-width-modulated signal; generating, in response to a representative signal related to the output signal, a filtered version of the representative signal; comparing the filtered version of the representative signal to the filtered version of the pulse-width-modulated signal; wherein isolating the power-supply output signal includes isolating the power-supply output node in response to an amplitude of the filtered version of the representative signal being greater than an amplitude of the filtered version of the pulse-width-modulated signal; and driving the power-supply output node in response to the amplitude of the filtered version of the representative signal being less than the amplitude of the filtered version of the pulse-width-modulated signal.

Example 18 includes the method of any of Examples 8-17, further comprising: comparing a magnitude of a representative signal related to the output signal to the signal magnitude; generating a comparison signal having a power-supply-node isolation value in response to the magnitude of the representative signal being greater than the signal magnitude; generating the comparison signal having a power-supply-node drive value in response to the magnitude of the representative signal being less than the signal magnitude; and latching the comparator signal in response to the comparison signal transitioning to the power-supply-node drive value.

Example 19 includes the method of any of Examples 8-18, further comprising: generating, in response to the pulse-width-modulated signal, a filtered version of the pulse-width-modulated signal, the signal magnitude equaling an average amplitude of the filtered version of the pulse-width-modulated signal; comparing a representative signal related to the output signal to the filtered version of the pulse-width-modulated signal; wherein isolating the power-supply output node includes isolating the power-supply output node in response to an amplitude of the representative signal being greater than the average amplitude of the filtered version of the pulse-width-modulated signal; and driving the power-supply output node in response to the amplitude of the representative signal being less than the average amplitude of the filtered version of the drive signal.

Example 20 includes the method of any of Examples 8-19, further comprising: comparing a magnitude of a representative signal related to the output signal to the signal magnitude; generating a comparison signal having a power-supply-node isolate value in response to the magnitude of the representative signal being greater than the signal magnitude; generating the comparison signal having a power-supply-node drive value in response to the magnitude of the representative signal being less than the signal magnitude; and latching the comparison signal in response to the comparison signal transitioning to the power-supply-node drive value and maintaining the power-supply-node drive value for at least one full period of the pulse-width-modulated signal.

Example 21 includes the method of any of Examples 8-20, further comprising: comparing a magnitude of a representative signal related to the output signal to the signal magnitude; generating a comparison signal having a power-supply-node isolate value in response to the magnitude of the representative signal being greater than the signal magnitude; generating the comparison signal having a power-supply-node drive value in response to the magnitude of representative signal being less than the signal magnitude; latching the comparison signal in response to the comparison signal transitioning to the power-supply-node drive value; and generating a version of the comparison signal from the comparison signal; and transitioning the version of the comparator signal to the power-supply-node drive value in response to the comparison signal transitioning to the power-supply-node drive value and in response to the pulse-width-modulated signal transitioning to an active value.

Example 22 includes the method of any of Examples 8-21, further comprising: generating a soft-start signal; generating an error signal in response to a difference between the soft-start signal and a feedback signal that is related to the output signal; and generating the pulse-width-modulated signal in response to the error signal.

From the foregoing it will be appreciated that, although specific embodiments have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the disclosure. Furthermore, where an alternative is disclosed for a particular embodiment, this alternative may also apply to other embodiments even if not specifically stated. Moreover, the components described above may be disposed on a single or multiple IC dies to form one or more ICs, these one or more ICs may be coupled to one or more other ICs. In addition, any described component or operation may be implemented/performed in hardware, software, firmware, or a combination of any two or more of hardware, software, and firmware. Furthermore, one or more components of a described apparatus or system may have been omitted from the description for clarity or another reason. Moreover, one or more components of a described apparatus or system that have been included in the description may be omitted from the apparatus or system. 

What is claimed is:
 1. A system, comprising: a power supply, including an input node configured to receive an input signal, an output node, a power-supply phase circuit configured to generate an output signal on the output node in response to the input signal, the output signal having a magnitude, a control circuit configured to drive the first power-supply phase circuit with a duty cycle, and a signal-drop-reducing circuit configured to disable the first power-supply phase circuit in response to the duty cycle corresponding to a signal magnitude that is lower than the magnitude of the output signal; and a load coupled to the output node of the power supply.
 2. The system of claim 1 wherein the power supply includes a buck converter.
 3. The system of claim 1 wherein the power supply includes a boost converter.
 4. The system of claim 1 wherein the power supply includes a buck-boost converter.
 5. The system of claim 1 wherein the power supply includes a SEPIC converter.
 6. The system of claim 1 wherein the power supply includes a OUK converter.
 7. The system of claim 1 wherein the load includes a microprocessor or a microcontroller.
 8. A method, comprising: generating a pulse-width-modulated signal having a duty cycle; and isolating a power-supply output node in response to the duty cycle corresponding to a signal magnitude that is less than a magnitude of an output signal on the power-supply output node.
 9. The method of claim 8 wherein generating the pulse-width-modulated signal includes generating the pulse-width-modulated signal in response to a difference between a reference signal and a feedback signal related to the output signal.
 10. The method of claim 8 wherein generating the pulse-width-modulated signal includes generating the pulse-width-modulated signal in response to a difference between a soft-start signal and a feedback signal related to the output signal.
 11. The method of claim 8 wherein isolating the power-supply output node includes isolating the power-supply output node in response to the duty cycle corresponding to a signal magnitude that is equal to a magnitude of an output signal on the power-supply output node.
 12. The method of claim 8, further comprising driving the power-supply output node in response to the pulse-width-modulated signal and in response to the duty cycle corresponding to a signal magnitude that is greater than the magnitude of the output signal.
 13. The method of claim 12 wherein: isolating the power-supply output node includes tri-stating a power-supply phase circuit; and wherein driving the power-supply output node includes activating the power-supply phase circuit.
 14. The method of claim 8, further comprising: comparing a representative signal related to the output signal to the signal magnitude; and wherein isolating the power-supply output node includes isolating the power-supply output node in response to a magnitude of the representative signal being greater than the signal magnitude.
 15. The method of claim 8, further comprising: generating, in response to the pulse-width-modulated signal, a filtered version of the pulse-width-modulated signal, the signal magnitude equaling an amplitude of the filtered version of the pulse-width-modulated signal; comparing a representative signal related to the output signal to the filtered version of the pulse-width-modulated signal; wherein isolating the power-supply output node includes isolating the power-supply output node in response to an amplitude of the representative signal being greater than the amplitude of the filtered version of the pulse-width-modulated signal; and driving the power-supply output node in response to the amplitude of the representative signal being less than the amplitude of the filtered version of the pulse-width-modulated signal.
 16. The method of claim 15 wherein the representative signal equals the output signal.
 17. The method of claim 8, further comprising: generating, in response to the pulse-width-modulated signal, a filtered version of the pulse-width-modulated signal, the signal magnitude equaling an amplitude of the filtered version of the pulse-width-modulated signal; generating, in response to a representative signal related to the output signal, a filtered version of the representative signal; comparing the filtered version of the representative signal to the filtered version of the pulse-width-modulated signal; wherein isolating the power-supply output signal includes isolating the power-supply output node in response to an amplitude of the filtered version of the representative signal being greater than an amplitude of the filtered version of the pulse-width-modulated signal; and driving the power-supply output node in response to the amplitude of the filtered version of the representative signal being less than the amplitude of the filtered version of the pulse-width-modulated signal.
 18. The method of claim 8, further comprising: comparing a magnitude of a representative signal related to the output signal to the signal magnitude; generating a comparison signal having a power-supply-node isolation value in response to the magnitude of the representative signal being greater than the signal magnitude; generating the comparison signal having a power-supply-node drive value in response to the magnitude of the representative signal being less than the signal magnitude; and latching the comparator signal in response to the comparison signal transitioning to the power-supply-node drive value.
 19. The method of claim 8, further comprising: generating, in response to the pulse-width-modulated signal, a filtered version of the pulse-width-modulated signal, the signal magnitude equaling an average amplitude of the filtered version of the pulse-width-modulated signal; comparing a representative signal related to the output signal to the filtered version of the pulse-width-modulated signal; wherein isolating the power-supply output node includes isolating the power-supply output node in response to an amplitude of the representative signal being greater than the average amplitude of the filtered version of the pulse-width-modulated signal; and driving the power-supply output node in response to the amplitude of the representative signal being less than the average amplitude of the filtered version of the drive signal.
 20. The method of claim 8, further comprising: comparing a magnitude of a representative signal related to the output signal to the signal magnitude; generating a comparison signal having a power-supply-node isolate value in response to the magnitude of the representative signal being greater than the signal magnitude; generating the comparison signal having a power-supply-node drive value in response to the magnitude of the representative signal being less than the signal magnitude; and latching the comparison signal in response to the comparison signal transitioning to the power-supply-node drive value and maintaining the power-supply-node drive value for at least one full period of the pulse-width-modulated signal.
 21. The method of claim 8, further comprising: comparing a magnitude of a representative signal related to the output signal to the signal magnitude; generating a comparison signal having a power-supply-node isolate value in response to the magnitude of the representative signal being greater than the signal magnitude; generating the comparison signal having a power-supply-node drive value in response to the magnitude of representative signal being less than the signal magnitude; latching the comparison signal in response to the comparison signal transitioning to the power-supply-node drive value; and generating a version of the comparison signal from the comparison signal; and transitioning the version of the comparator signal to the power-supply-node drive value in response to the comparison signal transitioning to the power-supply-node drive value and in response to the pulse-width-modulated signal transitioning to an active value.
 22. The method of claim 8, further comprising: generating a soft-start signal; generating an error signal in response to a difference between the soft-start signal and a feedback signal that is related to the output signal; and generating the pulse-width-modulated signal in response to the error signal. 